Visa källkod för Är cannabisanvändare farliga i trafiken?

[[kategori:Artiklar med videoklipp]]
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'''OBS!''' Målet med artikeln är inte att föra fram budskapet att man kan röka cannabis och sedan sätta sig i bilen, dvs köra påverkad. Det framgår tydligt att cannabis, likt de flesta andra psykoaktiva droger hämmar vissa neurologiska funktioner, bl.a sänks reaktionsförmågan en aning vid oväntade händelser. Man kan därtill missa trafikskyltar, ha svårare att hålla avståndet till andra bilar och linjer. Effekter som innebär risker i trafiken.

Artikelns syfte är att utvärdera hur hög risken är jämfört med nyktra förare, undersöka hur lång tid den negativa effekten finns och utreda hur denna effekten kan jämföras med alkohol, andra substanser och övriga risker/faror i trafiken.

Artikeln om [[cannabismetaboliternas psykoaktivitet]] går in djupare på blodkoncentration av THC och dess metaboliter och utvärderar vid vilken nivå som negativa effekter inte längre kan påvisas. Detta är intressant eftersom det i statistiken förekommer väldigt höga siffror för antalet trafikdödade som har THC-metaboliter i blodet. För icke insatta personer kan det tolkas som attt cannabis innebär en väldigt hög risk att råka ut för olyckor. Den kopplingen används även för att göra den tolkningen av förbudsförespråkare i debatter. Men man måste ha beaktande att cannabis är den vanligaste illegala drogen i samhället och därtill kan den även detekteras längst i urinen, upp till flera veckor längre tid än andra illegala droger. THC-COOH som är den mest framträdande metaboliten som detekteras i blod och urinprov är dock inte psykoaktiv.

==Argument från förbudsförespråkare==
Jan Ramströms bok "Skador av hasch och marijuana" från 2009 som gavs ut av det som då hette statens folkhälsoinstitut kan anses vara Sveriges officiella syn på vetenskapen vid tidpunkten. Boken har ett helt kapitel om cannabis och bilkörning (kapitel 16, sid 88). Klicka på referenslänken här<ref name=ramstrom>[http://www.folkhalsomyndigheten.se/pagefiles/12336/R-2009-13-Skador-av-hasch-o-marijuana.pdf Jan Ramström, Folkhälsoinstitutet - Skador av hasch och marijuana (2009)] (död länk, [https://www.foretagshalsor.se/sites/default/files/2020-10/Skador_av_hasch_och_marijuana.pdf backup finns här] och [https://www.yumpu.com/sv/document/read/20818738/skador-av-hasch-och-marijuana-statens-folkhalsoinstitut en extra här])</ref> och läs kapitlet i sin helhet för alla argument och källor, vars antal och omfattning inte kommer gås igenom i sin helhet här.

Detta är sammanfattningen:

{{quote|Experimentella studier visar hur cannabisrökning har en negativ påverkan på en rad mentala funktioner (psykomotoriska och kognitiva) som är viktiga för bilkörning.
Studier både i simulatorer och av körning på väg utan respektive med trafik visar hur cannabis redan i måttliga doser försämrar förmågan att köra bil.
En rad välgjorda epidemiologiska studier har bekräftat de experimentella studierna. Bland annat har man i flera studier visat hur förares halt av THC i blodet har stått i direkt relation till risken att vålla en trafikolycka. Den som intagit cannabis löper 3–7 gånger större risk än den som är opåverkad att orsaka en olycka, visar en välkänd översikt över sådana studier 
Såväl experimentella undersökningar som epidemiologiska studier visar att cannabis i kombination med alkohol, genom synergism, ger en farlig påverkan på bilföraren.
Vid försök att kartlägga de omständigheter som ökar sannolikheten för ”körning av motorfordon under cannabispåverkan” fann man bland annat följande faktorer:
*cannabisberoende/cannabismissbruk
*blandmissbruk 
*okunnighet om riskerna med cannabis i trafiken.|Jan Ramström, Folkhälsoinstitutet - Skador av hasch och marijuana<ref name=ramstrom> </ref>}}

I början av kapitlet påpekas att vanerökare är extra farliga eftersom de lagrar mycket mer THC i kroppen. Huruvida det stämmer och i vilken grad effekten är kvardröjande behandlas i artikeln om [[cannabismetaboliternas psykoaktivitet]].

{{quote|Inriktningen är här inte på de negativa effekter som dagligt eller nästan dagligt missbruk har på psykosocialt fungerande i allmänhet, utan främst på de omedelbara (och kvardröjande) effekter som cannabisruset har på mentala funktioner som är viktiga för bilkörningen, främst inom sfären kognitiva och psykomotoriska funktioner. Dessa effekter verkar vara ungefär desamma oavsett om cannabisrökaren är nybörjare eller vanerökare. För intensivrökaren tillkommer dessutom effekten av de i allmänhet betydligt högre doserna på grund av upprepad inlagring av THC vid fortsatt rökning. Vid avbrott kvarstår residualeffekter – speciellt på de kognitiva funktionerna – under betydligt längre tid för intensivrökaren.|Jan Ramström, Folkhälsoinstitutet - Skador av hasch och marijuana<ref name=ramstrom></ref>}}

Kapitlet handlar till stor del om genomgångar av stuidersom gjordes på 1970-, 1980- och 1990-talet. Få källor från 2000-talet tas upp.

Ramström ägnar en sida om forskning där piloter fått köra flygsimulator påverkade, naturligtvis med sämre utgång än under nyktert tillstånd. En studie från 1976 av Janowsky<ref>[http://www.ncbi.nlm.nih.gov/pubmed/1252201 Simulated flying performance after marihuana intoxication (Janowsky, 1976)]</ref> tas upp men vad som inte nämns är att den negativa effekten från cannabis kunde påvisas efter 2 timmar men hade försvunnit efter 4-6 timmar.

På flera ställen påpekas att dosen som användes i olika studier var för låg eller "måttlig" när en studie inte visar tillräckligt allvarliga risker. Ramström påpekar att man på senare tid i en nyare studie sett en risk med högre doser, vilket kan misstolkas av läsaren som att forskningen som nyss nämndes inte skall tas på allvar när en nyare visar något annat:

{{quote|När det gällde ett par andra funktioner, bland annat den viktiga reaktionstiden, var resultaten osäkra. Wilson och medarbetare (1993) har dock senare visat på ett tydligt samband mellan en mer realistisk dos cannabis (15–35 mg) och reaktionstiden.

...

Trots att piloterna endast tillfördes en låg dos THC (8 mg) uppvisade de tydliga funktionsförsämringar.

...

fortfarande mycket måttliga – doser THC (20 mg)

...

Kommentar till de äldre undersökningarna: Moskowitz (1985) drog bland annat slutsatsen att i stort sett vid alla ovan refererade undersökningar har man använt jämförelsevis låga doser av THC, vilket innebär att effekten av mer ”realistiska” doser kan vara avsevärt större.

...

I tre undersökningar (Hansteen, 1976; Klonoff, 1974; Attwood, et al., 1981) med bilkörning på bana utan trafik registrerades lätta till måttliga försämringar av körförmågan. I en av studierna undersöktes också körning i trafik med osäkert utfall. I samtliga fall användes dock låga eller mycket låga doser av THC. Robbe (1994) genomförde ett omfattande forskningsprogram vars tyngdpunkt var bilkörning dels på en avstängd väg, dels på en trafikerad motorväg. En undersökningsgrupp (som bestod av personer som rökte cannabis mer än en gång per månad men inte dagligen) fick pröva ut den optimala dosen för att bli ”hög” och utifrån genomsnittet av denna utprovning bestämdes den högsta försöksdosen till cirka 20 mg (300 mikrogram/kg kroppsvikt). Detta är sannolikt en relativt låg dos. Vanerökare kan konsumera 200 mg och ibland det dubbla på en dag (Moskowitz, 1985). Med de starka marijuanasorter som finns i dag kan en enda cigarett på 1 gram innehålla upp till 200 mg THC.|Jan Ramström, Folkhälsoinstitutet - Skador av hasch och marijuana<ref name=ramstrom> </ref>}}

Detta är inte en rättvis bild av doseringen. 10-20mg THC är en högst rimlig dos för den berusning som majoriteten av alla cannabisrökare vill uppnå. 20mg motsvarar 0.1g marijuana av de starkaste sorterna (med 20% THC), det är t.o.m något som många vanerökare helst hade delat upp i flera pipor under en längre tidsperiod istället för att röka allt med en gång, vilket sker i studierna. Men Ramström vill prompt påpeka att att man minsann kan röka 200mg per dag och därmed kan läsaren lockas att i huvudet börja mångdubbla riskerna som forskningarna visar. Eller vill man måhända att deltagarna ska drogas ner med 200mg för att detta skulle ge "rätt" resultat, dvs ett resultat som solklart pekar på att cannabis är extremfarligt? Det är knappast det realistiska fallet av rattonykterhet man vill återskapa om man endast väljer att titta på resultat från extrema doser.

Hur kan man påstå att "detta är sannolikt en relativt låg dos" när deltagarna själva rapporterade att det var "70% of the greatest ever experienced" ?

{{quote|In a a dose finding study by Robbe<ref name=robbe>[http://www.drugtext.org/Table/Influence-of-Marijuana-on-Driving/ Influence of Marijuana on Driving (Robbe, 1994)]</ref> 23 subjects who where all recreational users of THC indicated that they had achieved their desired psychological effect after smoking a mean dose of 300ug/kg (i.e indentical to smoking about 20 mg THC by a person of average weight). The range of this preferred dose varied between 194-524 ug/kg THC indicating considerable inter-individual variation.|Drugs, Driving and Traffic Safety<ref>[http://books.google.se/books?id=ZLv887ijMIUC&pg=PA494&lpg=PA494#v=onepage&q&f=false Drugs, Driving and Traffic Safety av Joris C. Verster, S. R. Pandi-Perumal, Jan G. Ramaekers, Johan J. de Gier (2009)]</ref>}}

{{quote|As mentioned, the aim of Robbe’s (1994) preliminary study was to determine the appropriate doses of cannabis to be used in the driving studies for recreational cannabis users. Twenty-four participants were given the opportunity to smoke a maximum of three cannabis cigarettes ad lib for up to 15 mins or until they had achieved the maximum psychological effect. The cigarettes contained on average 20 mg THC. Of the 23 participants, six consumed one cigarette, 13 smoked two, and four smoked all three cigarettes.

The amount of THC smoked did not differ for those people who smoked more than once per week versus those who smoked more than once a month but less than once a week. Participants reported their peak feelings of being ‘high’ to be on average 70% of the greatest ever experienced. The mean amount of cannabis smoked ad-lib (by body weight) was 308 mg/kg, and hence on the basis of these findings the maximum THC dose for the following experiments was set at 300 (mg/kg). While there are undoubtedly differences in the smoking experiences associated with smoking in normal social settings compared to smoking in the laboratory, the dose of cannabis used here represents the mean amount that was used by recreational users in a 15 min period.|Cannabis And Road Safety: A Review Of Recent Epidemiological, Driver Impairment, And Drug Screening Literature<ref>[http://www.monash.edu.au/miri/research/reports/muarc231.pdf Monash University Accident Research Centre 2004: Cannabis And Road Safety: A Review Of Recent Epidemiological, Driver Impairment, And Drug Screening Literature]</ref>}}

{{quote|In general, only a small amount of smoked cannabis (e.g. 2mg to 3mg of available THC) is required to produce a brief pleasurable high for the occasional user, and a single joint may be sufficient for two or three individuals. A heavy smoker may consume five or more joints per day, while heavy users in Jamaica, for example, may consume up to 420mg THC per day (Ghodse, 1986). In clinical trials designed to assess the therapeutic potential of THC, single doses have ranged up to 20mg in capsule form. In human experimental research, THC doses of 10mg, 20mg and 25mg have been administered as low, medium and high doses (Barnett et al 1985; Perez-Reyes et al 1982).|Australian Government: Department of Health and Ageing<ref>[http://www.health.gov.au/internet/main/publishing.nsf/Content/health-pubs-drug-cannab2-ch45.htm Australian Government: Department of Health and Ageing - The health and psychological consequences of cannabis use (1994)]</ref>}}

... Därmed kan man konstatera att forskarna kan sitt jobb och använder samma doser som en cannabisbrukare med stor sannolikhet också skulle använda.

==Forskning om cannabis och bilkörning==

1993 sammanställde U.S. Department of Transportation (den amerikanska motsvarigheten till trafikverket) en rapport om riskerna med cannabis och bilkörning:

{{quote|The major conclusions from the present program are summarized as follows:
* Current users of marijuana prefer THC doses of about 300 ug/kg to achieve their desired "high".
* It is possible to safely study the effects of marijuana on driving on highways or city streets in the presence of other traffic.
* Marijuana smoking impairs fundamental road tracking ability with the degree if impairment increasing as a function of the consumed THC dose.
* Marijuana smoking which delivers THC up to a 300 ug/kg dose slightly impairs the ability to maintain a constant headway while following another car.
* A low THC dose (100 ug/kg) does not impair driving ability in urban traffic to the same extent as a blood alcohol concentration (BAC) of 0.04g%.
* Drivers under the influence of marijuana tend to over-estimate the adverse effects of the drug on their driving quality and compensate when they can; e.g. by increasing effort to accomplish the task, increasing headway or slowing down, or a combination of these.
* Drivers under the influence of alcohol tend to under-estimate the adverse effects of the drug on their driving quality and do not invest compensatory effort.
* The maximum road tracking impairment after the highest THC dose (300 ug/kg) was within a range of effects produced by many commonly used medicinal drugs and less than that associated with a blood alcohol concentration (BAC) of 0.08g% in previous studies employing the same test.
* It is not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.|U.S. Department of Transportation: Marijuana And Actual Driving Performance (1993)<ref>[http://druglibrary.org/schaffer/misc/driving/driving.htm U.S. Department of Transportation: Marijuana And Actual Driving Performance (1993)]</ref>}}

En annan studie sponsrad av U.S. Department of Transportation från 1994 skriver om hur effekten från cannabis skiljer sig från alkohol när det gäller risktagande, hastighet m.m. :

{{quote|The foregoing comparisons might be misleading. THC's effects differ qualitatively from many other drugs, especially alcohol. For example, subjects drive faster after drinking alcohol and slower after smoking marijuana (Hansteen et aL, 1976; Casswell, 1979; Peck et al., 1986; Smiley et aL, 1987). Moreover, the simulator study by Ellingstad et al. (1973) showed that subjects under the influence of marijuana were less likely to engage in overtaking maneuvers, whereas those under the influence of alcohol showed the opposite tendency. Very importantly, our city driving study showed that drivers who drank alcohol overestimated their performance quality whereas those who smoked marijuana underestimated it. Perhaps as a consequence, the former invested no special effort for accomplishing the task whereas the latter did, and successfully. This evidence strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the former's users seem better able to compensate for its adverse effects while driving under the influence. Weil et aL (1968) were among the earliest authors who mentioned the possibility that marijuana users can actively suppress the drug's adverse effects. They presumed that THC's effects were confined to higher cortical functions without any general stimulatory or depressive effect on lower brain centers. According to them, the relative absence of neurological, as opposed to psychiatric, symptoms in marijuana intoxication suggests this possibility. More recently, Moskowitz (1985) concluded that the variety of impairments found after marijuana smoking could not be explained by decrements in sensory or motor functions which led him to hypothesize that some important central cognitive process is impaired by THC, without saying what it is. The recent discovery of abundant cannabinoid receptor concentrations in the cerebral cortex and the hippocampal formation corroborates these hypotheses and, with other findings to come, will certainly greatly enhance our understanding of the drug's psychopharmacological effects.|Influence of Marijuana on Driving, kapitel 9<ref name=robbe> </ref>}}

En av forskarna som arbetat längst med frågan om cannabis och trafikfaror, professor Alison Smiley släppte en rapport 1999<ref>Marijuana: On-road and driving simulator studies. (Smiley, 1999) I Kalant H., Corrigal W., Hall W., Smart R. (Eds.), The health effects of cannabis (sid. 173–191)</ref> som uppmärksammades av Science Daily:

{{quote|The safety hazards of smoking marijuana and driving are overrated, says University of Toronto researcher Alison Smiley. 
Recent research into impairment and traffic accident reports from several countries shows that marijuana taken alone in moderate amounts does not significantly increase a driver's risk of causing an accident -- unlike alcohol, says Smiley, an adjunct professor in the department of mechanical and industrial engineering. While smoking marijuana does impair driving ability, it does not share alcohol's effect on judgment. Drivers on marijuana remain aware of their impairment, prompting them to slow down and drive more cautiously to compensate, she says.

"Both substances impair performance," Smiley says. "However, the more cautious behaviour of subjects who received marijuana decreases the drug's impact on performance. Their behaviour is more appropriate to their impairment, whereas subjects who received alcohol tend to drive in a more risky manner."

Smiley, who has studied transportation safety for over 25 years, drew her results from a "metanalysis" of existing research into the effects of marijuana on driving ability, combined with traffic accident statistics in the United States and Australia. Previous studies showing stronger effects often combined "fairly hefty doses" by researchers with driving immediately after consumption, likely exaggerating the drug's effects, she believes.|Science Daily (1999-03-29)<ref>[http://www.sciencedaily.com/releases/1999/03/990325110700.htm Science Daily 1999-03-29: University Of Toronto Study Shows Marijuana Not A Factor In Driving Accidents]</ref>}}

En studie från 2000 blev uppmärksammad i media för att även den visar att alkohol är farligare än cannabis i trafiken.

{{quote|The new study was undertaken by the Transport Research Laboratory in Crowthorne, Berkshire, and confirms the results of a preliminary study more than a year ago.

...

In the study, cannabis significantly affected only one criterion, known as tracking ability. Volunteers found it more difficult to hold a constant speed and follow the middle of the road accurately while driving around a figure-of-eight loop. The TRL researchers point out in their draft report that this test requires drivers to hold their concentration for a short time, a task which is particularly badly affected by the intoxicating effects of cannabis.

However, volunteers drinking the equivalent of a glass of wine fared worse than those who had smoked a joint. Those who were given both alcohol and cannabis performed worse still, reinforcing the idea that alcohol has a cumulative effect when taken with other drugs.

But the study also found that drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate.

...

This cautious behaviour is in line with findings by other researchers. "Whereas alcohol promotes risk taking like fast speeds and close following, cannabis promotes conservative driving, but may cause attention problems and misperceptions of time," says Nicholas Ward, technical adviser to the Immortal project - a three-year European Union trial designed to quantify the crash risk drivers face after taking various drugs and medicines.|New Scientist (2002-03-20)<ref>[http://www.newscientist.com/article/dn2063-alcohol-impairs-driving-more-than-marijuana.html New Scientist 2002-03-20: Alcohol impairs driving more than marijuana]</ref>}}

Se även: [http://news.bbc.co.uk/2/hi/health/1068625.stm BBC 2000-12-13: Cannabis driving danger measured]

Studien gjordes på cannabisanvändare som använde cannabis minst en gång i veckan under de senaste 12 månaderna. Här är några citat från forskarnas sammanfattning:

{{quote|There was a reduction of average speed on simulated motorway driving when participants had the high or low doses of cannabis. This strongly suggests that the participants as drivers are aware of their impairment, but attempt to compensate for their impairment by driving more cautiously.

Also in the simulator trials, participants reacted more slowly to a pulling-out event when they had taken the low dose of cannabis, suggesting a similar compensatory action for the effects of cannabis impairment. However, when taking the high dose this effect was not significant. When considering the simulator tracking tasks, participants tended to drive less accurately on the left and right loops of the ‘figure of eight’ when they had been on the high cannabis dose. This suggests that they were unable to control their steering as well when under the influence of the high cannabis dose. This again confirms previous observations that cannabis adversely affects drivers tracking ability.

There is a variability in the results when considering the impairing effects of cannabis that has been observed by other researchers. The variability of drug effects on individuals is well recognised and this seems to be even more in evidence with cannabis than other with drugs. The failure to produce significant results on various driving performance measurements when compared to alcohol may be explained by the more variable effects of cannabis on participants.

The results of the driving related laboratory tests conducted in general did not produce statistically significant results. Although reaction times were found to increase with dose level, there was too much variability in the data for statistical significance. This suggests that there may be an effect on the reaction time of participants responding to hazards, but it is quite a small effect which would require a much larger sample to determine whether or not it was statistically significant. This again confirms earlier observations of the effects of cannabis on the various aspects of driver performance; the effect on reaction time being somewhat difficult to predict.

...

Overall, it is possible to conclude that cannabis has a measurable effect on psycho-motor performance, particularly tracking ability. Its effect on higher cognitive functions, for example divided attention tasks associated with driving, appear not to be as critical. Drivers under the influence of cannabis seem to compensate to some extent for the impairment, that they recognise, by reducing the difficulty of the driving task; e.g. by driving more slowly. In terms of road safety, it cannot be concluded that driving under the influence is not a hazard, as the effects on various aspects of driver performance are unpredictable. In comparison with alcohol however, the severe effects of alcohol on the higher cognitive processes of driving are likely to make this more of a hazard, particularly at higher blood alcohol levels.|Sexton (2000)<ref>[http://www.csdp.org/research/TRL477.pdf The influence of cannabis on driving (Sexton, 2000)]</ref>}}

INSERM (Institut national de la santé et de la recherche médicale) som är Frankrikes motsvarighet till FHI uttalar sig om cannabis i en omfattande utredning från 2001:

{{quote|Despite the assumption that it is dangerous to drive after or while using cannabis, even today it is still impossible to state, in the absence of reliable epidemiological studies, that there is a causal connection between cannabis use and road traffic accidents. The first difficulty epidemiologists are confronted with is establishing a control group. The other major difficulty is the absence of a synchronous relationship between the presence of cannabis (blood or urine) and its effects on behaviour. Δ9-THC level may actually be almost zero, and yet the harmful effect may last, or conversely, the metabolites may be detected well after all psychological effect or deterioration in abilities has disappeared. It is difficult, then, to classify subjects as those who are "exposed to the cannabis risk" and those who are not. Some teams have nevertheless attempted to establish a cannabis–accidents relationship by using an approach founded on an analysis of responsibility. A relative overrepresentation index of cannabis users among those responsible for accidents is used instead of an indicator of increased risk. The distinction between those responsible and not responsible should itself be made with caution once it has to be strictly independent of cannabis consumption and of the variables correlated with it (alcohol in particular). Relevant publications take into account these various biases as far as possible and confirm the importance of alcohol risk but fail overall to demonstrate any independent effect of cannabis on responsibility for accidental injury or death. Their results do suggest, however, that alcohol and cannabis combined represents a risk factor greater than that of alcohol alone. They also tend to show that the risk of responsibility increases at high Δ9-THC concentrations.

Overall, the results of different types of study do agree that the degree to which cannabis makes driving dangerous could depend on its modes of consumption, whether in a substantial quantity (elevated blood Δ9-THC concentrations) or mixed with alcohol. Beyond the remaining questions about the role of cannabis as a risk factor for accidents at the population level, substantial progress has been made in the observation system itself, with the development of biological media, appropriate thresholds and devices suitable for roadside use. Saliva tests are promising in this regard. This progress is sustained by the desire both to acquire knowledge and to take action in the sphere of psychoactive substances and traffic on the European level. Although a relationship may have been established between existing measures relating to alcohol and measures developing for other psychoactive substances, the scientific foundation, in the case of cannabis, still seems incomplete.|Cannabis: Effects of consumption on health<ref>[http://www.ncbi.nlm.nih.gov/books/NBK7131/ Cannabis: Effects of consumption on health (2001)]</ref>}}

Kanadensiska senatens utredning av cannabis från 2002 går också igenom riskerna med bilkörning:

{{quote|Given the difficulties of conducting reliable epidemiological studies on driving under the influence of cannabis, a number of authors have analyzed the probability of responsibility and the risk ratio involved in the use of cannabis. These studies distinguish between drivers who are responsible for accidents and those who are not. The former are the subjects and the latter the control group. Comparisons are then made of their intoxication to various substances. Clearly, placing drivers into the two categories of responsible/not responsible may depend on an investigator’s perception of whether or not psychoactive substances are present.

...

The study findings show that cannabis alone does not increase the likelihood of responsibility in an accident. However, most of the studies used a measurement of THC-COOH, an inactive metabolite that can remain in urine for several days. When the authors separated out THC alone, the risk ratio was slightly higher, even though it did not reach the required level of significance. In addition, as the concentration of THC increases, the more the ratio increases, once again suggesting a dose-response relationship. Furthermore, the cannabis and alcohol combination significantly increases risk. Without being able to draw any definite conclusions, there are some signs that their effects are in synergy and not merely additive.

Studies on injured drivers (Terhune (1982) and Hunter (1998)) have ratios somewhat higher than in the other studies on fatal accidents. According to Bates and Blakely (1999), the apparent reduction in the risk of a fatal accident stems from the fact that drivers under the influence of cannabis drive less dangerously, for example by reducing their speed.

...

Ramaekers et al. (2002), reported a meta-analysis on 87 controlled laboratory studies on the psychomotor effects of cannabis conducted by Berghaus et al. (1998). These authors found that the number of psychomotor functions linked to driving (following, reaction time, perception, hand-eye coordination, body balance, signal detection and divided and continuous attention) affected by THC reached a maximum during the first hour after smoking, and one to two hours after oral ingestion. The maximum figures were comparable to those obtained with an alcohol concentration equivalent to > 0.05 g/dl. The number of functions affected reached zero after three to four hours, and only higher doses continued to have an effect. The studies surveyed also showed that THC concentration in the blood is highly correlated to psychomotor effects: a concentration of between 14 ng/ml and 60 ng/ml affected between 70% and 80% of tasks

More recently, after surveying the studies carried out in recent years, the reports prepared by INSERM and the International Scientific Conference on Cannabis reached largely similar conclusions: cannabis affects reaction time where choice is involved, road tracking, shared attention and continuous attention, as well as memory processes, but does not significantly affect simple reaction time or visual or eye-movement functions.

...

It is interesting to recall that one of the first driving studies on the road was conducted for the Le Dain Commission. In this study, on a closed track, 16 subjects were each given the 4 following preparations: placebo, marijuana 21 and marijuana 88 μg/kg THC and a dose of alcohol equivalent to BAC 0.07. The tests were conducted immediately after use and three hours later. The subjects were to complete six circuits of the track (1.8 km) with manoeuvres involving slowing down while going forward and backwards, maintaining a trajectory and weaving through cones. The alcohol and heavy dose of marijuana decreased driver performance in tests conducted immediately after use. At the heavy cannabis dose, drivers drove more slowly. On the second test, the differences were less clear.

When the results of this study are compared to those conducted more recently using much more sophisticated methods, it can be seen that the results are remarkably similar. Thus the following was observed:

* Lateral control: this is the variable that is most sensitive to the effects of THC, but the effects are variable, depending on the dose and time; only heavy doses significantly affected lateral control over the vehicle. In comparison, alcohol has a greater effect on vehicle lateral control and speed (linked variables)
* Speed control: in almost all cases, the use of cannabis significantly decreases speed
* Risk-taking: in addition to decreasing speed, it is generally found that there is an increase in distance between vehicles among marijuana users, and less of a tendency to pass or attempt dangerous manoeuvres
* Decision time: this variable is particularly important in actual driving situations. The results do not appear to be very consistent. Smiley suggests that reaction time is unaffected when the subjects are told that they need to respond rapidly, whereas on the other hand, when the obstacles are completely unexpected, the subjects who used cannabis do not perform as well
* Combined effects of alcohol and cannabis: when the researchers checked the effects of the two substances, the combined effects of cannabis and alcohol were systematically greater than alcohol alone or, even more so, than cannabis alone.

Lastly, with low doses, subjects had the impression that their driving was not as good as observers felt it was, which was not necessarily the case with higher doses, where the perceptions of both the drivers and the observers agreed.|Cannabis : Our Position For A Canadian Public Policy<ref name=kanada>[http://www.parl.gc.ca/Content/SEN/Committee/371/ille/rep/repfinalvol1part4-e.htm#Chapter%208 Cannabis : Our Position For A Canadian Public Policy - Report Of The Senate Special Committee On Illegal Drugs, kapitel 8 (2002)]</ref>}}

Man belyser även problemet med att se tydliga samband, man har följande exempel på när kritik riktats mot tidigare forskningar som överdrivit faran med cannabis i trafiken:

{{quote|...the Canadian Police Association told us:

Driving while intoxicated by drugs impairs judgment and motor coordination. In one study involving aircraft 10 licensed pilots were given one marijuana joint containing 19 milligrams of THC - a relatively small amount. Twenty-four hours after smoking the joint, they were tested in a flight simulator. All 10 of the pilots made errors in landing and one missed the runway completely.

Two weeks later, Dr. John Morgan of the City University of New York Medical School said in reference to the same study:

A California-based scientist named Jerome Yesavage wrote the study. It was done in the early 1980s, I think, and it attracted enormous attention. … Doctor Yesavage's study … was completely uncontrolled. … As you all have heard, it is difficult to control for marijuana use. When Doctor Yesavage was funded by the federal government to repeat the study with the simple controls that others and I had suggested, they were unable to show any impact of marijuana use after four hours in a similar group of people. Therefore, I believe that the truth is that marijuana use will impact airplane and driving simulators and to some degree driving performance for three hours to four hours after use; however there is no sustained impact. Any impact is relatively minor.

Making reference to Robbe’s work, which we will be examining in greater detail in this chapter, Professor Morgan added:

A Dutch scientist who has for years worked on driving experiments found that marijuana using drivers have a little difficulty staying right in the middle of the road. That is most sensitive test. If you smoke marijuana, you tend to weave a little bit more than completely sober people do. That is important, although there have been no studies to show that that amount of weaving had a gross impact on driving ability. 

The Dutch scientist included in his report that the amount of weaving was approximately the same in individuals consuming very small amounts of alcohol, very small doses of bensodiazopenes and very small doses of antihistamines.

On the same day, Professor Kalant of the University of Toronto responded as follows:

Dr. Morgan referred to some experimental studies this morning. A number of studies, reviewed by Dr. Smiley in the report of the World Health Organization Committee on Health Effects of Cannabis, indicate a fair measure of agreement on what the predominant effects on driving are. The lane control, as Dr. Morgan mentioned, is impaired. The person does not steer as accurately. In addition, there was slower starting time and slower braking time. There was decreased visual search. In other words, when you drive, you must monitor for sources of danger to both sides and not just ahead of you. There was decreased monitoring, decreased recognition of danger signals. The effects were synergistic with those of alcohol. The one favourable thing about cannabis compared with alcohol was that there was less aggressiveness in the cannabis smokers than in the drinkers, so they were less likely to pass dangerously or to speed. Nevertheless, driving ability was impaired not just by weaker, poorer steering control, but also by less alertness to unexpected things that might happen and pose a hazard. 

I will not go into the statistics of actual field studies of the involvement of cannabis in driving accidents. However, I would like to say that a number of studies have shown that there has been evidence of cannabis presence in the blood or the urine of people who have been stopped for impaired driving who did not have alcohol present.

As we can see, and as was the case with respect to the effects and consequences on the health of users, there are divergent opinions about the interpretation of studies and their meaning in connection with the specific effects of marijuana on driving.|Cannabis : Our Position For A Canadian Public Policy<ref name=kanada> </ref>}}

Vi klipper även med en bit från sammanfattningen:

{{quote|The Committee feels it is quite likely that cannabis makes users more cautious, partly because they are aware of their deficiencies and compensate by reducing speed and taking fewer risks. However, because what we are dealing with is no longer the consequences on the users themselves, but the possible consequences of their behaviour on others, the Committee feels that it is important to opt for the greatest possible caution with respect to the issue of driving under the influence of cannabis.

Given what we have seen, we conclude the following: 

*Between 5% and 12% of drivers may drive under the influence of cannabis; this percentage increases to over 20% for young men under 25 years of age;
*Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving. Cannabis leads to a more cautious style of driving. However it has a negative impact on decision time and trajectory. This in itself does not mean that drivers under the influence of cannabis represent a traffic safety risk;
*A significant percentage of impaired drivers test positive for cannabis and alcohol together. The effects of cannabis when combined with alcohol are more significant than is the case for alcohol alone;
*Despite recent progress, there does not yet exist a reliable and non intrusive rapid roadside testing method;
*Blood remains the best medium for detecting the presence of cannabinoids;
*Urine cannot screen for recent use;
*Saliva is promising, but rapid commercial tests are not yet reliable enough;
*The visual recognition method used by police officers has yielded satisfactory results; and
*It is essential to conduct studies in order to develop a rapid testing tool and learn more about the driving habits of cannabis users.|Cannabis : Our Position For A Canadian Public Policy<ref>Sammanfattningen av [http://www.parl.gc.ca/Content/SEN/Committee/371/ille/rep/summary-e.htm Cannabis : Our Position For A Canadian Public Policy - Report Of The Senate Special Committee On Illegal Drugs (2002)]</ref>}}

En studie från 2005 där försökspersoner fick cannabis oralt visar också på att dessa har förmågan att själva känna av när de är påverkade och de kände sig inte bekväma med att köra bil och skulle inte vilja göra det annat än i nödfall:

{{quote|'''Subjective effects and willingness to drive'''
The volunteers reported the subjective effects and willingness to drive on an visual analog scale (0 to 10 cm). When compared to placebo, obvious cannabis influence was observed under almost all treatments. These differences were also statistically significant (Kruskal-Wallis test, p < 0.0001). On the whole, pair-wise comparisons were also statistically significant for each treatment versus placebo (Tukey test, p < 0.05). However, most pair-wise comparisons between treatments were not significant. We found a moderate degree of acceptance when an absolutely vital demand was addressed to the participants (e.g., do you agree to drive an ill child to the hospital?). On the other hand, we found a strong refusal when the subjects were asked a question of less importance (e.g., do you agree to drive a friend to a party?). Robbe has previously reported that the willingness to drive decreased with increasing doses of cannabis. He also found that the willingness to drive was greatest for urgent trips and increased with time. These results suggest that the subjects were able to balance the importance of the trip against the risk of having an accident. The participants were aware of the effects of the drug and reported a strong feeling of "high". The self-reported intoxication was more intense after ingestion of the highest dose. Liguori et al. have also shown that self-report ratings of "high" and "drug potency" as well as the feeling of impairment increased with the smoking dose.|Menetrey (2005)<ref name=assessment>[http://jat.oxfordjournals.org/content/29/5/327.full.pdf Assessment of Driving Capability Through the Use of Clinical and Psychomotor Tests in Relation to Blood Cannabinoids Levels Following Oral Administration of 20 mg Dronabinol or of a Cannabis Decoction Made with 20 or 60 mg d9-THC (Menetrey, 2005)]</ref>}}

I en artikel från 2009 sammanfattas några olika forskares angivelser för vilka nivåer i THC som innebär risker:

{{quote|Clearly, cannabis produces impairment in neurocognitive and psychomotor skills necessary for safe driving; however, defining the relationship between THC blood concentrations and performance decrements has been challenging. Numerous early studies failed to find a significant increase in accident risk when cannabinoids were present in blood or urine. In some of these, the presence of the non-psychoactive THCCOOH metabolite, rather than THC, defined recent cannabis use. Other limitations included long intervals between accident occurrence and blood collection leading to false negative cannabinoid results, and limited numbers of cases positive for cannabis only. More recently, Drummer et al. conducted a study in 3398 fatally injured drivers to determine the effect of cannabis on accident culpability. Using a validated method of classifying drivers as culpable or non-culpable, the investigators found that accident risk significantly increased in drivers with measurable blood THC concentrations (no LOQ provided) when compared to drug-free drivers (odds ratio (OR) 2.7, 95% confidence interval (CI) 1.0 – 7.0). When THC concentrations were greater than 5 ng/mL, culpability increased (OR 6.6, 95% CI 1.5 – 28.0). This odds ratio is comparable to that observed with a blood alcohol concentration of 0.15 g%.

Recent experimental laboratory research proposed impairment limits of 2 – 5 ng/mL serum THC (approximately 1 – 2.5 ng/mL whole blood) after observing behavioral impairment in a majority of participants with serum THC concentrations within the suggested limits in tasks relating to driving skills. Others suggested that a serum THC between 7 and 10 ng/mL (approximately 3.5 – 5.0 ng/mL whole blood) was indicative of impairment, similar to a BAC of 0.05% based on a meta-analysis of multiple toxicological studies. But it is difficult to apply these limits in the field, because THC concentrations decrease rapidly after cannabis smoking, even in frequent users, from high peak concentrations (100 to 400 ng/mL depending upon the individual, cannabis potency, and smoking parameters) to levels of 1 to 10 ng/mL in a few hours. The time required to obtain biological specimens after automobile or industrial accidents often exceeds 3 h, frequently leading to negative THC tests. Drug concentrations in the majority of DUID cases are 1 – 2 ng/mL. If a 5 ng/mL whole blood limit had been the law, 77 – 90% of apprehended drivers recently using cannabis in Sweden from 1995 – 2004 would not have been prosecuted. Conversely, as the present study shows, some individuals may display concentrations well over 1 ng/mL many days after last cannabis exposure.|Karschner (2009)<ref>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2784185/?tool=pmcentrez Do Δ9-Tetrahydrocannabinol Concentrations Indicate Recent Use in Chronic Cannabis Users? (Karschner, 2009)]</ref>}}

2011 släpptes en studie som granskade statistik från 16 av USA:s delstater där cannabislagarna lättats. Den visade att antalet trafikdödade minskar när cannabis till viss del ersätter alkohol som berusningsdrog.

{{quote|The passage of state medical-marijuana laws is associated with a subsequent drop in the rate of traffic fatalities, according to a newly released study by two university professors.

The study — by University of Colorado Denver professor Daniel Rees and Montana State University professor D. Mark Anderson — found that the traffic-death rate drops by nearly 9 percent in states after they legalize marijuana for medical use. The researchers arrived at that figure, Rees said, after controlling for other variables such as changes in traffic laws, seat-belt usage and miles driven. The study stops short of saying the medical-marijuana laws cause the drop in traffic deaths.

...

Rees said the main reason for the drop appears to be that medical-marijuana laws mean young people spend less time drinking and more time smoking cannabis. Legalization of medical marijuana, the researchers report, is associated with a 12-percent drop in the alcohol-related fatal-crash rate and a 19-percent decrease in the fatality rate of people in their 20s, according to the study.|Denver Post 2011-11-30<ref>[http://www.denverpost.com/news/marijuana/ci_19437417 Denver Post 2011-11-30: Report shows fewer traffic fatalities after states pass medical-pot laws]</ref>}}

Professor Daniel Rees vid University of Colorado och professor D. Mark Anderson vid Montana State University beskriver med egna ord vad de kommit fram till:

{{quote|The current study draws on data from a variety of sources to explore the effects of legalizing medical marijuana. Using data from the National Survey on Drug Use and Health (NSDUH), we find that the use of marijuana by adults in Montana and Rhode Island increased after medical marijuana was legalized. Although opponents of legalization argue that it encourages recreational use among teenagers (Brady et al. 2011; O’Keefe and Earleywine 2011), we find no evidence that the use of marijuana by minors increased.

Using data from the Fatality Analysis Reporting System (FARS) for the period 1990-2009, we find that traffic fatalities fall by nearly 9 percent after the legalization of medical marijuana. This effect is comparable in magnitude to those found by economists using the FARS data to examine other policies. For instance, Dee (1999) found that increasing the minimum legal drinking age to 21 reduces fatalities by approximately 9 percent; Carpenter and Stehr (2008) found that mandatory seatbelt laws decrease traffic fatalities among 14- through 18-year-olds by approximately 8 percent.

Why does legalizing medical marijuana reduce traffic fatalities? Alcohol consumption appears to play a key role. The legalization of medical marijuana is associated with a 6.4 percent decrease in fatal crashes that did not involve alcohol, but this estimate is not statistically significant at conventional levels. In comparison, the legalization of medical marijuana is associated with an almost 12 percent decrease in any-BAC fatal crashes per 100,000 licensed drivers, and an almost 14 percent decrease in high-BAC fatal crashes per 100,000 licensed drivers.

The negative relationship between legalization of medical marijuana and traffic fatalities involving alcohol is consistent with the hypothesis that marijuana and alcohol are substitutes. In order to explore this hypothesis further, we examine the relationship between medical marijuana laws and alcohol consumption using data from the Behavioral Risk Factor Surveillance System and The Brewer’s Almanac. We find that the legalization of medical marijuana is associated with decreased alcohol consumption, especially by 20- through 29-year-olds. In addition, we find that legalization is associated with decreased beer sales, the most popular alcoholic beverage among young adults (Jones 2008).|Medical Marijuana Laws, Trafﬁc Fatalities, and Alcohol Consumption (Rees & Anderson 2011)<ref>[http://ftp.iza.org/dp6112.pdf Medical Marijuana Laws, Trafﬁc Fatalities, and Alcohol Consumption (Rees & Anderson 2011)]</ref>}}

Som alternativ förklaring nämner man förvisso att det kan vara så att den mer stigmatierade milön runt cannabis gör att personerna som väljer cannabis istället för alkohol gör det i hemmet istället för i krogmiljön, varpå ingen behöver köra bilen hem och råka ut för trafikolyckan.


I en uppföljande studie från 2013 stärks teorin om färre dödade i trafiken till följd av att alkoholanvändare vänder sig till cannabis:

{{quote|The estimates in Table 13 offer additional support for the hypothesis that legalization reduces traffic fatalities through its impact on alcohol consumption. They are uniformly negative and often statistically significant at conventional levels. Moreover, the relationship between legalization and alcohol consumption appears to be strongest among young adults, the group for whom the relationship between legalizationand traffic fatalities was strongest.

For instance, among 20–29-year-olds, legalization is associated with a 5.3 percent (.031/.589) reduction in the probability of having consumed alcohol in the past month, a 19.6 percent (.011/.056) reduction in the probability of having consumed 60 or more drinks, and a 10.6 percent (2.40/22.71) reduction in the number of drinks consumed (conditional on having had at least one drink). During the period 1990–2010, almost one-fourth of individuals killed in traffic accidents, and more than one-third of individuals killed in traffic accidents involving alcohol, were between the ages of 20 and 29.

Respondents to BRFSS are also asked how many times in the past month they binge drank, defined as having five or more alcoholic beverages on an occasion. The estimates in Table 13 suggest that the legalization of medical marijuana leads to sharp reductions in binge drinking, a form of alcohol abuse considered to have “especially high social and economic costs” (Naimi et al. 2003, p. 70). Among 18- and 19-year-olds, legalization is associated with a 9.4 percent (.018/.192) reduction in the probability of binge drinking in the past month; among 40–49-year-olds, legalization is associated with an 8.8 percent (.013/.147) reduction in this probability. Among 20–29-year-olds, legalization is associated with a 7.4 percent (.012/.163) reduction in the probability of binge drinking at least twice in the past month.

...

Evidence from simulator and driving-course studies provides a potential explanation for why substituting marijuana for alcohol could lead to fewer traffic fatalities. These studies show that alcohol consumption leads to an increased risk of collision (Kelly, Darke, and Ross 2004; Sewell, Poling, and Sofuoglu 2009). Even at low doses, drivers under the influence of alcohol tend to underestimate the degree to which they are impaired (MacDonald et al. 2008; Marczinski, Harrison, and Filmore 2008; Robbe and O’Hanlon 1993; Sewell, Poling, and Sofuoglu 2009), drive at faster speeds, and take more risks (Burian, Liguori, and Robinson 2002; Ronen et al. 2008; Sewell, Poling, and Sofuoglu 2009). In contrast, simulator and driving-course studies provide only limited evidence that driving under the influence of marijuana leads to an increased risk of collision, perhaps as a result of compensatory driver behavior (Kelly, Darke, and Ross 2004; Sewell, Poling, and Sofuoglu 2009).|Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption (Anderson, Hansen & Rees, 2013)<ref>[http://www.jstor.org/discover/10.1086/668812 Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption (Anderson, Hansen & Rees, 2013)] [http://gatton.uky.edu/faculty/sandford/391_f13/marijuana.pdf pdf-version]</ref>}}


En studie från 2015<ref>[http://www.drugandalcoholdependence.com/article/S0376-8716%2815%2900314-2/abstract Cannabis effects on driving lateral control with and without alcohol (Hartman et. al 2015)]</ref> med 18 förare som fick dricka alkohol eller inandas vaporiserad cannabis visade att gränsnivån för drograttfylla som satts i Colorado och Washington för THC i blodet (5ug/l) ger effekter som är långt mindre allvarliga än effekterna från Alkohol vid gränsvärdet 0.8 promille. Det krävdes 13.1ug/l THC för att nå samma nivå. Alkohol i kombination med cannabis var värre än var drog för sig i nivåer under de satta gränserna för rattfylleri.

{{quote|A new study, funded in part by the federal government, suggests alcohol has a more extreme impact on drivers than marijuana.

Researchers said alcohol "significantly increased lane departures/minimum and maximum lateral acceleration; these measures were not sensitive to cannabis." Researchers also concluded Cannabis-influenced drivers "may attempt to drive more cautiously to compensate for impairing effects, whereas alcohol-influenced drivers often underestimate their impairment and take more risk." 

...

Researchers said "alcohol, but not marijuana, increased the number of times the car actually left the lane and the speed of the weaving."|CNN 2015-06-24<ref>[http://edition.cnn.com/2015/06/24/politics/marijuana-study-drivers-impact/○3 CNN 2015-06-24: Fed study: Booze impact greater than pot on driving]</ref>}}

2016 publicerades ännu en studie<ref>[http://onlinelibrary.wiley.com/doi/10.1002/jat.3295/abstract Cannabis effects on driving longitudinal control with and without alcohol (Hartman, 2016)]</ref> som undersökte cannabis effekter på bilförare. Man kunde se att personerna som fått THC körde saktare, höll sig under fartgränserna och ökade avståndet till framförvarande fordon. Man såg även att personer med både alkohol och THC i blodet körde saktare än personer med enbart alkohol i blodet

Den amerikanska motororganisationen American Automobile Association (AAA) dömer 2016 ut THC-tester som verktyg för att bedömma om en förare är trafikfarlig, det är helt enkelt inte möjligt att använda uppmätta THC-nivåer på samma sätt som alkoholutandningsprover för att avgöra berusningsgraden:

{{quote|The study commissioned by AAA's safety foundation said it's not possible to set a blood-test threshold for THC, the chemical in marijuana that makes people high, that can reliably determine impairment. Yet the laws in five of the six states automatically presume a driver guilty if that person tests higher than the limit, and not guilty if it's lower.

...

"There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment in the same manner we do alcohol," said Marshall Doney, AAA's president and CEO. "In the case of marijuana, this approach is flawed and not supported by scientific research."

...

There's no science that shows drivers become impaired at a specific level of THC in the blood. A lot depends upon the individual. Drivers with relatively high levels of THC in their systems might not be impaired, especially if they are regular users, while others with relatively low levels may be unsafe behind the wheel.|CBS News (2016-06-10)<ref>[http://www.cbsnews.com/news/tests-for-driver-impairment-by-marijuana-flawed-aaa/ CBS News 2016-06-10: Tests for driver impairment by marijuana flawed: AAA]</ref>}}


En studie från 2020 visar att deltagarna som fick cannabis ansåg sig nedsatta att köra även vid moderata doser, och känslan av nedsatthet satt kvar även när studiens tester inte längre kunde uppmäta någon nedsatt effekt jämfört med placebo efter 4-5 timmar<ref>[https://jamanetwork.com/journals/jama/fullarticle/2773562 Effect of Cannabidiol and d9-Tetrahydrocannabinol on Driving PerformanceA Randomized Clinical Trial, (Arkell, 2020)]</ref>
Det är ganska tvärt emot alkohol där en person som druckit för mycket ändå känner sig redo och konfident att han/hon inte har nedsatt körförmåga, vilket fortfarande morgonen efter en fest finns kvar, trots det kör många hem med en enligt lagen för hög halt alkohol i blodet


En stor metaanalys från Australien som publicerades 2021 visar att blod och salivtester som i vissa fall används för att fastställa om någon kör drograttfull är dåliga, eftersom det inte finns någon korrelation mellan THC-halt och graden av nedsatt förmåga, speciellt inte hos reguljära brukare:

{{quote|No significant biomarker-performance relationships were observed in ‘regular’ (weekly or more often) cannabis users (p’s>0.10), although the analyses were less robust. Blood and oral fluid THC concentrations are relatively poor indicators of cannabis/THC-induced impairment.

...
While some significant biomarker–performance relationships were observed in occasional cannabis users, none were detected in regular cannabis users. These findings suggest per se limits are unlikely to be effective in distinguishing between impaired and unimpaired (or not-meaningfully-impaired) regular cannabis users. This compromises the validity of per se limits in general; that is, it is inappropriate to have a regulatory framework that lacks validity in a key target demographic (i.e., regular cannabis users). Several factors might account the observed differences between regular and other cannabis users, including that: (1) regular cannabis users appear to be less sensitive to the impairing effects of THC than occasional cannabis users (McCartney et al., 2021b; Colizzi and Bhattacharyya, 2018); and (2) THC-related biomarkers (i.e., from prior cannabis use) can persist in biological matrices for prolonged periods of time (Karschner et al., 2009).|McCartney (2021)<ref>[https://doi.org/10.1016/j.neubiorev.2021.11.004 Are blood and oral fluid d9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis (McCartney, 2021)]</ref>}}

==Risk- och oddskvoter för cannabis och alkohol==
Ett odds (oddstal) är sannolikheten för att en viss händelse skall inträffa dividerad med sannolikheten för att den inte skall inträffa. Ett odds kan variera från 0 till oändligheten. En oddskvot utgår från 1 som normalläge, där oddset inte avviker från det man jämför med. En riskkvot (RR) är väldigt lik en oddskvot (OR) vid låga avvikelser från 1, men differensen ökar ju högre kvoten är. OR-talet visar alltid en större differens mellan grupperna än RR. Läs mer om oddskvoter och riskkvoter i artikeln "Risk och odds – hur man räknar med händelser"<ref>[http://ltarkiv.lakartidningen.se/2002/temp/pda24274.pdf Risk och odds – hur man räknar med händelser (Läkartidningen, Nr 8 2002)]</ref>.


1999 gjorde australiska forskare en undersökning av 2500 förare som omkommit i bilolyckor. Man undersökte förekomsten av olika droger och beräknade även andelen som ansågs vara vållande till olyckan. Man kunde se att nyktra förare var vållande i 52.8% av fallen. För cannabis (d9-THC) <2.1ng/ml var det 36.8% och >2.1ng/ml 66.7% . För alkohol 0.5–0.79 promille 87.5% , för 0.8–0.14 promille 91.7% . Man kunde med andra ord se att låga nivåer THC innebar en lägre risk än att köra bil nykter:

{{quote|...The present study found no signiﬁcant relationship between THC and culpability. While a larger number of injured drivers tested positive for THC compared with other culpability studies (Williams et al., 1985; Terhune et al., 1992), their culpability rate was no higher than that of the drug-free group. As in the present study, these past studies found that a higher percentage of drug-free drivers were culpable for the crash compared with drivers who tested positive for THC only. However, the results failed to reach statistical signiﬁcance. Moreover, some studies (Warren et al., 1981; Garriott et al., 1986) were unable to determine a culpability rate for THC alone due to the small number of drivers testing positive. Another limitation in some past studies has been the failure to separate drivers positive for THC with those only positive for the inactive metabolite THC-acid. For example, Drummer (1994) found that drug-free drivers had a higher culpa-bility rate than drivers positive for cannabinoids. How-ever, the difference was not statistically signiﬁcant. Drummer also acknowledged that only THC-acid was found in the majority of cases, and that results were usually from urine samples, not blood. There were only ten drivers who tested positive for THC alone, a number too small to obtain accurate culpability rates (Drummer, 1999). The present study thus has important implications in clarifying the relationship between THC and culpability, with the results conﬁrming previous research suggesting that THC alone may not increase crash risk. Moreover, unlike some previous studies (e.g. Drummer, 1994), drivers in this study who tested positive for THC-acid only (which does not suggest recent use of marijuana) were excluded from the culpability analyses.|The prevalence of alcohol, cannabinoids, benzodiazepines and stimulants amongst injured drivers and their role in driver culpability<ref>[http://http.usmanscience.com/12bio/Supplementary/Human_Biology/Canabis/AAP2.pdf The prevalence of alcohol, cannabinoids, benzodiazepines and stimulants amongst injured drivers and their role in driver culpability (Longo, 1999)]</ref>}}

Kanadensiska senatens rapport från 2002 citerar Jan Ramaekers (Professor vid Universiteit Maastrich) uppgifter<ref>Performance impairment and risk of motor vehicle crashes after cannabis use (Ramaekers, 2002) i Pelc, I. (ed.) International Scientific Conference on Cannabis, Bryssel, sid 81.</ref> som i sin tur har inhämtat data från andra forskares rapporter, se källanvisningen för varje riskkvot.

{{quote|The following table, which is reproduced from the Ramaekers et al. report (2002) for the International Scientific Conference on Cannabis summarizes the results of various studies. It should be pointed out that the probability of responsibility for drivers showing traces of cannabis (D9THC and/or D9THC–COOH, whether measured in blood or urine) is compared to the responsibility of drivers involved in an accident not testing positive to any substance (including alcohol). The risk ratio for drivers not testing positive to any substances is 1.0 and is used as a point of comparison to determine the statistical significance of observed change in the risk level of impaired drivers. When the reference value is above the statistical confidence level of 95%, the obvious conclusion is that the drug is 95% associated with an increased risk of responsibility.|Cannabis : Our Position For A Canadian Public Policy<ref name=kanada> </ref>}}

{| class="wikitable"
|-
! Substans !! Riskkvot !! Källa
|-
| Alkohol ||5.4 ||Terhune & Fell (1982), U.S.
|-
| THC ||2.1 ||Terhune & Fell (1982), U.S.
|-
| || &nbsp; ||
|-
| Alkohol ||5.0 ||Williams et al. (1985), U.S.
|-
| THC eller THC-COOH ||0.2 ||Williams et al. (1985), U.S.
|-
| Alkohol & THC eller THC-COOH ||8.6 ||Williams et al. (1985), U.S.
|-
| || &nbsp; ||
|-
| Alkohol ||7.4 ||Terhune et al. (1992), U.S.
|-
| THC ||0.7 ||Terhune et al. (1992), U.S.
|-
| Alkohol & THC || 8.4 ||Terhune et al. (1992), U.S.
|-
| || &nbsp; ||
|-
| Alkohol ||5.5 ||Drummer (1994), Australia
|-
| THC-COOH ||0.7 ||Drummer (1994), Australia
|-
| Alkohol & THC-COOH ||5.3 ||Drummer (1994), Australia
|-
| || &nbsp; ||
|-
| Alkohol ||6.8 ||Hunter et al. (1998), Australia
|-
| THC <= 1.0 ng/ml ||0.35 ||Hunter et al. (1998), Australia
|-
| THC 1.1-2.0ng/ml ||0.51 ||Hunter et al. (1998), Australia
|-
| THC >2ng/ml ||1.74 ||Hunter et al. (1998), Australia
|-
| THC-COOH 1-10 ng/ml ||0.69 ||Hunter et al. (1998), Australia
|-
| THC-COOH 11-20 ng/ml ||1.04 ||Hunter et al. (1998), Australia
|-
| THC-COOH 21-30 ng/ml ||0.87 ||Hunter et al. (1998), Australia
|-
| THC-COOH >30 ng/ml ||1.62 ||Hunter et al. (1998), Australia
|-
| Alkohol & THC ||11.5 ||Hunter et al. (1998), Australia
|-
| || &nbsp; ||
|-
| Alkohol ||3.2 ||Lowenstein & Koziol-McLain (2001), U.S
|-
| THC-COOH ||1.1 ||Lowenstein & Koziol-McLain (2001), U.S
|-
| Alkohol & THC-COOH ||3.5 ||Lowenstein & Koziol-McLain (2001), U.S
|-
| || &nbsp; ||
|-
| Alkohol ||5.7 ||Drummer et al. (2001) & Swann (2000), Australia
|-
| THC ||3.0 ||Drummer et al. (2001) & Swann (2000), Australia
|-
| THC >5ng/ml ||6.4 ||Drummer et al. (2001) & Swann (2000), Australia
|-
| THC-COOH ||0.8 ||Drummer et al. (2001) & Swann (2000), Australia
|-
| Alkohol & THC ||19 ||Drummer et al. (2001) & Swann (2000), Australia
|}

En litteraturstudie från 2003 redovisar ett antal olika uppskattningar som gjorts:

{{quote|'''The Use Of Responsibility Analysis Or Estimation Of Culpability To Determine The Role Of Drugs In Crashes'''
In the absence of a separate control group (as used in the assessment of crash probability with alcohol as described above) an alternative of a 'culpability index' is currently being employed in drug studies. The basic construct is first to formulate a means of determining the responsibility or culpability of a driver involved in a crash. There have been several means of constructing this 'culpability index' and this must be done with each of the accident cases by observers who have no information as to the drug status of each driver. The responsibility (or culpability) ratio is then determined as the proportion of drug-bearing drivers who were determined to be culpable, to the non-drug bearing drivers who were deemed to be culpable. The null hypothesis predicts a culpability ratio of 1.00 (ie, the drug has had no causal relationship with crashes). To date there have been six studies employing this technique (two of which have involved the re-analysis of earlier generated data). These are briefly outlined below:

*1. Warren and others re-analysed the data of Cimbura and found a culpability index for cannabis of 1.7, the same as that found for alcohol. However, the original data comprised a total of 484 drivers and pedestrians, 3.7% of whom were positive for cannabis. However, 88% of these people were also positive for alcohol. This left a very small number from which to assess a culpability ratio for cannabis alone.
*2. Terhune also has previously collected data independently re-analysed to estimate a culpability ratio. All BACs over 0.10% were judged significantly more culpable than the drug-free group. The cannabis group also had a higher culpability ratio than the drug-free group, but this was only marginally significant (58.8% vs 34.4%). This estimation was also compromised by the small sample size for cannabis only (n 17). The cannabis plus alcohol group was analysed separately.
*3. Donelson began a very ambitious project but was unfortunately thwarted by funding problems which precluded the complete analysis of the collected data. However, a random sample of 415 cases was analysed. The results cautiously suggested a finding consistent with those of Warren et al. and Terhune above.
*4. Williams et al. in a study involving 440 cases, demonstrated as in the above studies that alcohol had a higher culpability ratio compared with culpable drug-free drivers (92% vs 71%). However, those drivers in whom only cannabis was detected were less likely to be responsible for the crashes (53% vs 71%).
*5. Terhune et al. reported a very comprehensive study involving 1 882 cases. They found that alcohol was the dominant drug in fatal crashes, although the basic focus of their research was to describe the effect of drugs other than alcohol. They reported that fully 40% of the drivers had only alcohol in their systems and another 11% had alcohol combined with drugs. Among the drivers with BACs at or above 0.10% (n 625) their responsibility rate: ... was an extraordinary 94%, well above that found for any other single substance. Of cannabis, the authors stated that while cannabinoids were detected in 7% of the drivers, the psychoactive agent THC was found in only 4%. Of the drivers with only one substance in their system, only 1.1% had cannabis alone, either as the THC the psychoactive compound or had the inactive metabolite carboxy THC. The presence of the inactive metabolite and the absence of detectable THC infers less recent ingestion of cannabis assuming an efficient analysis. The THC only drivers had a responsibility rate below that of the drug-free driversÑie. as with the study by Williams et al. (1985) they were considered to be less likely to have been a cause of the crash than the drug-free drivers. The report also indicated the range of THC concentrations found in the blood. There were 109 cases of THC alone; of these, 22.9% contained what the authors called a 'trace' ie. 1 to 2 nanograms THC per millilitre of blood (ng/ml); 69.7% contained 'low' concentrations between 3 to 19 ng/ml; and 7.3% contained a 'high' concentration of equal to or greater than 20 ng/ml.

*6. Drummer reported a study of 1 045 fatalities in New South Wales, Victoria and Western Australia and used the technique of responsibility analysis (culpability index). As with other studies, the dominant drug was alcohol, being found overall in 36% of all driver fatalities, 33% of which were over the legal limit of 0.05g%. Cannabis was found in 11% of cases of which 56% (n 63) also contained alcohol (mean BAC 0.16 g% ± 0.08g%). There was no significant difference in the BAC of the alcohol only drivers and those with alcohol plus cannabis. Assessment of the culpability ratio by Drummer provided the same result as those of Williams et al. and Terhune et al; there was a trend to a decrease in relative risk when either THC or the metabolite carboxy THC was measured in blood or urine. The relative risk was 0.6 relative to drug-free drivers, although this was not significant statistically.|Cannabis And Road Safety: An Outline Of The Research Studies To Examine The Effects Of Cannabis On Driving Skills And On Actual Driving Performance<ref>[http://www.ukcia.org/research/driving/02.php Cannabis And Road Safety: An Outline Of The Research Studies To Examine The Effects Of Cannabis On Driving Skills And On Actual Driving Performance]</ref>}}

En Nederländsk studie från 2004<ref>[http://igitur-archive.library.uu.nl/phar/2006-0802-202323/movig_04_psychoactivesubstanceuseandtheriskof.pdf Psychoactive substance use and the risk of motor vehicle accidents (Movig, 2004)]</ref> angav en oddskvot på 1.22 för cannabis, enligt författarna en ickesignifikant ökning. Problemet med denna studien är man inte tog hänsyn till skillnaden mellan d9-THC och metaboliten THC-COOH som är kvarvarande i upp till flera veckor i urinet, därav testade många nyktra personer positivit för cannabis. Resultatet bör inte användas i debatten för att visa något annat än att just effekten av metaboliterna innebär en obetydlig riskökning.


2005 släpptes en fransk studie som gick igenom trafikolyckor med dödlig utgång visade att risken var drygt dubbelt så stor för den som rökt cannabis än den som var nykter. Rapporten uppmärksammades f.ö av Sveriges Radio som rapporterade att den väckte förvåning och bestörtning då risken visade sig vara mycket högre för alkohol<ref>[http://sverigesradio.se/sida/artikel.aspx?programid=83&artikel=750754 Sveriges Radio 2005-12-09: Alkohol värre än cannabis vid bilkörning]</ref>

{{quote|Positive cannabis detection was associated with increased risk of responsibility (odds ratio 3.32, 95% confidence interval 2.63 to 4.18). A significant dose effect was identified; the odds ratio increased from 2.18 (1.22 to 3.89) if 0< Δ9-tetrahydrocannabinol < 1 ng/ml to 4.72 (3.04 to 7.33) if Δ9-tetrahydrocannabinol 5 ng/ml.

...

The prevalence of cannabis (2.9%) estimated for the driving population is similar to that for alcohol (2.7%). At least 2.5% (1.5% to 3.5%) of fatal crashes were estimated as being attributable to cannabis, compared with 28.6% for alcohol (26.8% to 30.5%).|Cannabis intoxication and fatal road crashes in France: population based case-control study<ref>[http://www.bmj.com/content/331/7529/1371 Cannabis intoxication and fatal road crashes in France: population based case-control study (Laumon, 2005)]</ref>}}

Tabell med oddskvoter från franska rapporten. Högst kvot = högst risk. Promille för alkohol är mätt i blodet:

{| class="wikitable"
|-
! Substans !! Oddskvot (OR)
|-
|d9-THC < 1 ng/ml || 2.18
|-
|d9-THC 1 - 2 ng/ml || 2.54
|-
|d9-THC 3 - 4 ng/ml || 3.78
|-
|d9-THC >= 5 ng/ml || 4.72
|-
|Alkohol < 0.5 promille || 3.41
|-
|Alkohol 0.5 - 0.8 promille || 8.0
|-
|Alkohol 0.8 - 1.2 promille || 9.32
|-
|Alkohol 1.2 - 2.0 promille || 15.0
|-
|Alkohol >= 2.0 promille || 41.8
|}

2012 släppte kanadensiska forskare en rapport där man studerat nio studier med totalt 50000 människor i världen som skadats allvarligt eller avlidit i bilkrasher. Man kommer fram till att risken att orsaka en olycka under påverkan av cannabis är dubbelt så hög som för nyktra människor. Det gäller upp till 3 timmar efter att drogen intagits.

{{quote|Drivers who use cannabis up to three hours before driving are twice as likely to cause a collision as those not under the influence of drugs or alcohol, says a Canadian study. This is because cannabis impairs brain and motor functions needed for safe driving, the researchers suggest.

...

The researchers conclude that despite the increased risk posed by cannabis to car drivers, alcohol remains the substance most often present in crashes. The observed association between alcohol and crash risk is more significant than that for cannabis, the study says.|BBC (2012-02-10)<ref>[http://www.bbc.co.uk/news/health-16961112 BBC 2012-02-10: Cannabis drivers 'twice as likely to cause car crash']</ref>}}

Forskarnas egna kommentarer:

{{quote|We selected nine studies in the review and meta-analysis. Driving under the influence of cannabis was associated with a significantly increased risk of motor vehicle collisions compared with unimpaired driving (odds ratio 1.92 (95% confidence interval 1.35 to 2.73); P 0.0003); we noted heterogeneity among the individual study effects (I2 81). Collision risk estimates were higher in case-control studies (2.79 (1.23 to 6.33); P 0.01) and studies of fatal collisions (2.10 (1.31 to 3.36); P 0.002) than in culpability studies (1.65 (1.11 to 2.46); P 0.07) and studies of non-fatal collisions (1.74 (0.88 to 3.46); P 0.11).|Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis<ref>[http://www.bmj.com/content/344/bmj.e536 Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis (Asbridge, 2012)]</ref>}}

En Dansk studie från 2013<ref name=dansk>[http://www.ncbi.nlm.nih.gov/pubmed/23867258 Risk of severe driver injury by driving with psychoactive substances (Hels, 2013)] </ref> undersökte data från flera europeiska länder och kom fram till att alkohol är associerad med högst risk och cannabis med lägst risk för allvarliga olyckor:

{{quote|The highest risk of the driver being severely injured was associated with driving positive for high concentrations of alcohol (≥0.8g/L), alone or in combination with other psychoactive substances. For alcohol, risk increased exponentially with blood alcohol concentration (BAC). The second most risky category contained various drug-drug combinations, amphetamines and medicinal opioids. Medium increased risk was associated with medium sized BACs (at or above 0.5g/L, below 0.8g/L) and benzoylecgonine. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs. For male drivers, the risk of being severely injured by driving with any of the psychoactive substances was about 65% of that of female drivers. For each of the substance groups there was a decrease in the risk of severe driver injury with increasing age. It is concluded that among psychoactive substances alcohol still poses the largest problem in terms of driver risk of getting injured.|Risk of severe driver injury by driving with psychoactive substances (Hels, 2013)<ref name=dansk></ref>}}

Amerikanska forskare visade i en studie från 2013<ref>[http://www.sciencedirect.com/science/article/pii/S0001457513003527 Drug use and fatal motor vehicle crashes: A case-control study (Li, 2013)]</ref> att cannabis var associerad med lägst oddskvot för dödlig utgång i jämförelse med andra olagliga droger. Oddskvoten var 1.83 för cannabis, 3.03 för narkotiska droger (opiater), 3.57 för stimulanter och 4.83 för depressanter.


En Norsk metaanalys från 2013<ref name=elvik>[http://www.ncbi.nlm.nih.gov/pubmed/22785089 Risk of road accident associated with the use of drugs: a systematic review and meta-analysis of evidence from epidemiological studies (Elvik, 2013)]</ref> analyserade data från 66 olika studier som utvärderat olagliga droger och läkemedels risker vid bilkörning. Efter att man justerat för bias (forskares tendens att inte publicera studier som inte visar någon signifikant risk) så kom forskaren fram till att cannabis endast kunde associeras vid en liten riskökning, dock ej signifikant. Oddskvoten var 1.06 för en trafikolycka och 1.25 för dödlig utgång. I jämförelse med andra droger så var oddskvoten för dödlig utgång 1.44 för opiater, 2.30 för benzodiazepiner, 1.32 för antidepressanter, 2.96 för kokain, 4.46 för amfetamin, 2.60 för sömnmedlet Zopiklon (Imovane). Antihistaminers risknivå på 1.12 och penicillin på 1.12 var jämförbara med cannabis.


En amerikansk studie från 2014 visar att cannabis inte inverkar på oddskvoterna. När man justerade för påverkande demografiska faktorer (ålder, kön, etnicitet) så försvanna alla signifikanta samband, vilket förvånade forskarna:

{{quote|Although drugs other than alcohol do contribute to crash risk, we found that such a contribution depends on the type of drug under consideration. Somewhat unexpected was the finding that although marijuana’s crude OR indicated a significant contribution to fatal crash risk, once it was adjusted by the presence of alcohol and drivers’ demographics, marijuana’s OR was no longer significant among either sober or drinking drivers. This finding is somewhat surprising because, as reviewed by Sewell et al. (2009), most experimental and epidemiological studies (e.g., the DRUID project (Bernhoft, 2011; Hargutt et al., 2011) ) have reported at least a modest impairing effect of marijuana on driving. Citing MacDonald and colleagues (2008), Sewell et al. (2009) argue that a possible explanation for such a modest outcome is that, unlike alcohol drinkers, marijuana users tend to overestimate their level of impairment, which could induce them to apply compensatory strategies that would make them drive more cautiously. However, our results showing no increase in relative risk for a fatal crash associated with marijuana should be interpreted with caution. It is impossible to determine the time of marijuana use relative to the crash in the FARS database, and the contribution of d9-tetrahydrocannabinol (THC) to crash risk may become significant only among recent users (Ramaekers et al., 2004). Also, we used any measured presence of marijuana in the FARS, but only medium or high concentrations affect driving (Ménétrey et al., 2005). Furthermore, the excessive delays in the collection of some biological samples in the FARS file may have reduced the number of marijuana-positive results and diluted the contribution of marijuana to fatal crash risk.|Drugs and Alcohol: Their Relative Crash Risk (Romano, 2014)<ref>[http://www.jsad.com/jsad/article/Drugs_and_Alcohol_Their_Relative_Crash_Risk/4892.html Drugs and Alcohol: Their Relative Crash Risk (Romano, 2014)]</ref>}}


En studie från Nya Zealand som publicerades 2014 analyserade blodprover från 1046 trafikdödade förare. Man såg att personer med alkohol i blodet gav en oddskvot (OR) på 13.7. Kombinationen alkohol och cannabis (THC) gav en OR på 6.9. Personer som enbart hade använt cannabis hade en mycket svag OR på 1.3. Forskarna upptäckte även att en högre THC-nivå gav OR 1.0, vilket innebär att risken är jämförbar med helt nyktra förare:

{{quote|There was only a weak positive association between cannabis use (with no other drug) and culpability (OR 1.3, CI 95% 0.8-2.3). Furthermore, the OR for drivers with blood tetrahydrocannabinol (THC) concentrations greater than 5ng/mL was lower (OR 1.0, CI 95% 0.4-2.4) than drivers with blood THC concentrations less than 2ng/mL (OR 3.1, CI 95% 0.9-10). This is inconsistent with results reported by other studies where a significant increase in crash risk was found with blood THC levels greater than 5ng/mL.|The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs. (Poulsen, 2014)<ref>[http://www.ncbi.nlm.nih.gov/pubmed/24636874 The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs. (Poulsen, 2014)]</ref>}}


Den Amerikanska trafiksäkerhetsmyndigheten "National Highway Traffic Safety Administration" (NHTSA) som ingår i trafikdepartementet släppte 2015<ref name=comptonberning>[http://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf Drug and Alcohol Crash Risk (Compton & Berning, 2015)]</ref> en rapport som är en av de mest omfattande som någonsin gjorts. Data samlades in från drygt 3000 förare som varit inblandade i trafikolyckor och jämfördes med en kontrollgrupp på 6000 förare. Man hade även alkoholutandningsprov från 10221 förare, salivprov från 9285 förare och blodprov från 1764 förare. Förutom cannabis så analyserade studien riskerna för alkohol och andra droger. Cannabispositiva ansågs vara personer som hade d9-THC i blodet (dvs, inte inaktiva metaboliter). Man beräknade fram en riskökning på 1.25 (25% högre sannolikhet att vara inblandad i en trafikolycka) för cannabis. Men det intressanta är att när man justerade för demografiska variabler såsom ålder, kön och etnicitet samt användning av alkohol så raderades ökningen av riskoddsen totalt. Forskarna menar att anledningen till att vi tidigare sett höga riskodds kan vara att cannabis röks främst av unga män, som man vet är mer riskbenägna i trafiken.

{{quote|This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs shown in Table 3 is not found after adjusting for these demographic variables. This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC.

...

Table 5 adjusts the odds ratios by both demographic variables and the presence of alcohol. When the effect of alcohol is removed, the odds ratios decline further (except for a non-significant increase for narcotic analgesics). As was described above, there was no difference in crash risk for marijuana (THC)-positive drivers who were also positive for alcohol than for marijuana (THC)-positive drivers with no alcohol, beyond the risk attributable to alcohol.

...

This study of crash risk found a statistically significant increase in unadjusted crash risk for drivers who tested positive for use of illegal drugs (1.21 times), and THC specifically (1.25 times). However, analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs. This finding indicates that these other variables (age, gender ethnicity and alcohol use) were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and with THC. This study found a statistically significant association between driver alcohol level and crash risk both before and after adjustment for demographic factors. These findings were generally consistent with similar analyses conducted in prior crash risk studies.|Compton & Berning (2015)<ref name=comptonberning></ref>}}


En metastudie från 2016<ref>[http://www.ncbi.nlm.nih.gov/pubmed/26878835 The effects of cannabis intoxication on motor vehicle collision revisited and revised. (Rogeberg, 2016)]</ref> där man undersökte resultaten från 21 studier och replikerade en tidigare metastudie som inkluderade 9 studier gav en riskkvot (meta-regression odds ratio) på 1.22. Forskarna skriver att studier med höga riskkvoter även hade kvalitetsbrister gällande metoder och justering för andra faktorer än cannabis, exempelvis att man inte justerade/kontrollerade för alkoholpåverkan.

==Jämförelser==
===Cannabis som läkemedel===
Om man får 20mg THC om dagen sublingualt i form av läkemedel, kan man då köra bil som vanligt? Läkemedelsindustriföreningen i Sverige skriver följande om Sativex i FASS (uppslagsverk för läkemedel) som används av i princip alla läkare i Sverige:

{{quote|'''Körförmåga och användning av maskiner'''
* Du får inte köra bil eller använda verktyg eller maskiner när du påbörjar behandlingen med Sativex, förrän en stabil daglig dos har fastställts.
* Sativex kan orsaka sömnighet och yrsel, vilket kan leda till nedsatt omdömesförmåga och inverka på förmågan att utföra uppgifter som kräver skärpt uppmärksamhet. Enstaka fall av kortvarig medvetslöshet har också rapporterats.
* När du har vant dig vid Sativex och din dosering är stabil, bör du ändå inte köra bil eller använda verktyg eller maskiner om Sativex orsakar effekter som sömnighet eller yrsel, som kan försämra din förmåga att utföra sådana aktiviteter. Om du är osäker på hur du reagerar på Sativex, ska du inte köra bil eller hantera maskiner.
* Du är själv ansvarig för att bedöma om du är i kondition att framföra motorfordon eller utföra arbeten som kräver skärpt uppmärksamhet. En av faktorerna som kan påverka din förmåga i dessa avseenden är användning av läkemedel på grund av deras effekter och/eller biverkningar. Beskrivning av dessa effekter och biverkningar finns i andra avsnitt. Läs därför all information i denna bipacksedel för vägledning. Diskutera med din läkare eller apotekspersonal om du är osäker.|FASS<ref>[http://www.fass.se/LIF/produktfakta/artikel_produkt.jsp?NplID=20101019000051&DocTypeID=7&UserTypeID=2 FASS om Sativex]</ref>}}

Om man omvandlar rekommendationen till växtdrogen cannabis så skulle det kunna se ut såhär:

Kör inte bil om du är nybörjare, det är man innan man blivit van vid effekternas karaktärer och lärt sig hantera eller kompensera för cannabis negativa effekter på uppmärksamheten och reaktionsförmågan. Kör aldrig bil så länge du känner dig påverkad (vilket brukar innebära 3-5 timmar efter att du har rökt).

En studie från 2015<ref>[http://www.ncbi.nlm.nih.gov/pubmed/25208898 Drug-resistant MS spasticity treatment with Sativex add-on and driving ability. (Freidel, 2015)]</ref> av forskare från Tyskland, undersökte om MS-patienters förmåga att köra bil förändrades av medicineringen med Sativex. Man kom fram till att förmågan i stort var oförändrad.

Man har genom olika regressionsstudier även undersökt om lagar som tillåter medicinsk marijuana leder till fler dödsfall i trafiken. En studie från 2017<ref>[https://doi.org/10.2105/AJPH.2016.303577 US Traffic Fatalities, 1985–2014, and Their Relationship to Medical Marijuana Laws (Santaella-Tenorio, 2017)]</ref> visar att införandet av medicinsk marijuana i olika delstater i USA tvärtom ledde till en minskning av dödsfallen. Ett mastersarbete från 2019<ref>[https://krex.k-state.edu/dspace/handle/2097/39647 Marijuana legalization and road safety: a panel study of US States (Young, 2019)]</ref> som tittade över ett större tidsspann kunde inte finna några signifikanta ökningar eller minskningar, delstaterna följde samma utvecklingsmönster som liknande delstater som inte tillät medicinsk cannabis.

===Sömnbrist===
Jämförelser med alkohol finns det gott om bland studierna som reciteras här ovan och man kan se att cannabis inte ger samma allvarliga riskökning. Men finns det andra saker man kan jämföra med? Saker som inte många anser vara förknippade med höga risker, men som faktiskt är farligare än att röka cannabis?

CNN rapporterar att sömnbrist är lika allvarligt som att dricka alkohol innan man kör bil. Man jämför med en blodalkoholhalt på 0.5 promille, dvs över den svenska gränsen för rattfylleri.

{{quote|In a study published this week in the British journal Occupational and Environmental Medicine, researchers in Australia and New Zealand report that sleep deprivation can have some of the same hazardous effects as being drunk.Getting less than 6 hours a night can affect coordination, reaction time and judgment, they said, posing "a very serious risk."

Drivers are especially vulnerable, the researchers warned. They found that people who drive after being awake for 17 to 19 hours performed worse than those with a blood alcohol level of .05 percent. That's the legal limit for drunk driving in most western European countries, though most U.S. states set their blood alcohol limits at .1 percent and a few at .08 percent.

The study said 16 to 60 percent of road accidents involve sleep deprivation. The researchers said countries with drunk driving laws should consider similar restrictions against sleep-deprived driving.

The British Medical Association warned that there are other problems associated with sleep deprivation beyond impaired motor skills. People who get too little sleep may have higher levels of stress, anxiety and depression, and may take unnecessary risks.|CNN (2000-09-20)<ref>[http://articles.cnn.com/2000-09-20/health/sleep.deprivation_1_sleep-deprivation-impairment-researchers?_s=PM:HEALTH CNN 2000-09-20: Sleep deprivation as bad as alcohol impairment, study suggests]</ref>}}

Forskarna noterar även att när man kommer över 17-19 timmar utan sömn ger det samma effekt som med alkohol skulle räknas som grov rattfylla:

{{quote|After 17–19 hours without sleep, corresponding to 2230 and 0100, performance on some tests was equivalent or worse than that at a BAC of 0.05%. Response speeds were up to 50% slower for some tests and accuracy measures were significantly poorer than at this level of alcohol. After longer periods without sleep, performance reached levels equivalent to the maximum alcohol dose given to subjects (BAC of 0.1%). |Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication<ref>[http://oem.bmj.com/content/57/10/649.full Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication (Williamson, 2000)]</ref>}}

Att köra bil efter att ha varit vaken 17 timmar är inget extremt, för en person som vaknar klockan 6 på morgonen skulle det motsvara klockan 23. De flesta av oss har förmodligen gjort detta ett flertal gånger.

Att köra bil efter att ha sovit mindre än 6 timmar är också väldigt vanligt, hur många i morgontrafiken har fått sina 8 timmar sömn tror ni? Man fastnade lite för länge framför datorn eller TV:n igår, somnade efter midnatt men ska gå upp, duscha, äta frukost och vara på jobbet klockan 7?

{{quote|NTF Stockholms län har nyligen genomfört en stickprovundersökning bland privatbilister och yrkesförare. Resultatet visar att 52 procent av de tillfrågade privatförarna kör trötta några gånger om året eller ännu oftare. Siffrorna för de tillfrågade yrkesförarna är ännu högre: Hela 63 procent säger sig köra när de är trötta och 23 procent gör det så ofta som någon gång i månaden.

...

Ungefär 20 procent av alla trafikolyckor i Sverige kan relateras till trötthet. 50 människor dör i trötthetsrelaterade olyckor och betydligt fler skadas svårt.|NTF: Mer än hälften är ibland trötta när de kör bil<ref>[http://www.ntf.se/Stockholm/default.asp?RecID=30273 NTF: Mer än hälften är ibland trötta när de kör bil]</ref>}}

En japansk studie visar att sömn under 6 timmar ger oddsration (OR) 8.02, och 6-7 timmars sömn (OR) 6.28<ref>[http://www.sciencedirect.com/science/article/pii/S0386111213000149 Short sleep duration, sleep disorders, and traffic accidents (Komada, 2013)] (siffrorna refererar till artikeln "Questionnaire-based evidence of association between sleepiness while driving and motor vehicle crashes that are subjectively not caused by falling asleep" av samma forskare.)</ref>

===Tala i mobiltelefon===
Att tala i mobilen när man kör bil är lika farligt som att ha 0.8 promille alkohol i blodet enligt en studie från 2006.

Oddskvoten beräknades till 5.36 vilket är mycket högre än vad de flesta studierna anger för cannabis:

{{quote|"We found that people are as impaired when they drive and talk on a cell phone as they are when they drive intoxicated at the legal blood-alcohol limit” of 0.08 percent, which is the minimum level that defines illegal drunken driving in most U.S. states, says study co-author Frank Drews, an assistant professor of psychology. “If legislators really want to address driver distraction, then they should consider outlawing cell phone use while driving.”

Psychology Professor David Strayer, the study's lead author, adds: “Just like you put yourself and other people at risk when you drive drunk, you put yourself and others at risk when you use a cell phone and drive. The level of impairment is very similar.”

“Clearly the safest course of action is to not use a cell phone while driving,” concludes the study by Strayer, Drews and Dennis Crouch, a research associate professor of pharmacology and toxicology.

...

The study reinforced earlier research by Strayer and Drews showing that hands-free cell phones are just as distracting as handheld cell phones because the conversation itself – not just manipulation of a handheld phone – distracts drivers from road conditions.|University of Utah<ref>[http://unews.utah.edu/old/p/062206-1.html Drivers on Cell Phones Are as Bad as Drunks - University of Utah News Release: June 29th, 2006]</ref>}}

{{quote|When driving conditions and time on task were controlled for, the im-pairments associated with using a cell phone while driving can be as profound as those associated with driving while drunk.

...

the estimated odds ratio of an accident for cell phone drivers was 5.36, a relative risk similar to the estimates obtained by Zandor et al. (2000) for drivers with a blood alcohol level of 0.08% wt/vol.|A Comparison of the Cell Phone Driver and the Drunk Driver<ref>[http://www.distraction.gov/download/research-pdf/Comparison-of-CellPhone-Driver-Drunk-Driver.pdf A Comparison of the Cell Phone Driver and the Drunk Driver]</ref>}}

En annan studie visar att risken för att krocka var 4 gånger högre om föraren pratade i mobiltelefon än om föraren inte gjort det.<ref>[http://www.nejm.org/doi/full/10.1056/NEJM199702133360701 Association between Cellular-Telephone Calls and Motor Vehicle Collisions]</ref>

Att skriva eller titta på mobilskärmen när man kör bilen ger sannolikt högre riskodds än att bara ha den i  handen mot örat under telefonsamtal som dessa studierna (innan pekskärmarnas intåg) varit inriktade på.

I många länder är det totalförbjudet att prata i mobiltelefon under körningen, i andra länder tillåts handsfree. I Sverige är den exakta bestämmelsen att ''"Vid färd på väg med ett motordrivet fordon får föraren ägna sig åt aktiviteter som användning av mobiltelefon och annan kommunikationsutrustning endast om det inte inverkar menligt på förandet av fordonet. Föraren får inte använda denna utrustning på ett sådant sätt att han eller hon håller den i handen"''.

"Men jag använder ju headset" invänder kanske många nu, och ja. Då uppfyller man det som krävs rent lagligt. Men flera studier visar att det inte minskar risken. (Strayer & Johnston, 2001; Strayer, Drews, & Johnston, 2003 samt Wikipedia<ref>[http://en.wikipedia.org/wiki/Mobile_phones_and_driving_safety Wikipedia: Mobile phones and driving safety]</ref>. Lagtexten hanterar numera risken med kommunikationsutrustning som hålls i handen. Men det missar en sak. Att risken även handlar om själva samtalet över kommunikationsutrustningen, att det kräver ens koncentration på en nivå som gör att den inte kan spendera uppmärksamhet på att se faror i trafiken.

En studie från 2016<ref>[https://doi.org/10.1016/j.trf.2016.01.007 Imagery-inducing distraction leads to cognitive tunnelling and deteriorated driving performance (Briggs, 2016)]</ref> visar att samtalet tar hjärnans fokus bort från bilkörningen och ökar reaktionstiden med en sekund.

{{quote|Dr Graham Hole, senior lecturer in psychology at the University of Sussex, said: "A popular misconception is that using a mobile phone while driving is safe as long as the driver uses a hands-free phone.

"Our research shows this is not the case. Hands-free can be equally distracting because conversations cause the driver to visually imagine what they're talking about. This visual imagery competes for processing resources with what the driver sees in front of them on the road."
...
"You are 0.98 of a second slower to respond to hazards if you're on a hands-free mobile phone than if you're not."
...
Drivers distracted by hands-free phone conversations also focused on a small area of road and failed to spot hazards even when they looked at them, the study published in the journal Transportation Research revealed.

"Eye tracking shows that their eyes are falling on the hazard but they're not reacting," said Dr Hole. "The eyes are there but the brain's away."|Independent 2016-06-08<ref>[https://www.independent.co.uk/tech/talking-while-driving-is-incredibly-dangerous-even-when-using-handsfree-new-study-finds-a7070486.html Independent 2016-06-08: Talking while driving is incredibly dangerous, even when using hands-free, new study finds]</ref>}}

Studien såg inte samma samband med att lyssna på radio eller ha samtal i bilen.

När det gäller röststyrning i bilar så publicerades två studier 2015<ref>[https://unews.utah.edu/up-to-27-seconds-of-inattention-after-talking-to-your-car-or-smart-phone/ University of Utah 2015-10-22: Up to 27 seconds of inattention after talking to your car or smartphone]</ref> av forskare vid University of Utah där man testade hur förare reagerade på olika former av röstkontrollerade infotainment och mobila/automobila system. Det framgick att reaktionstiden kan bli försämrad upp till en halvminut efter användning. Man kunde rangordna olika bilmodeller, samt fastslog att alla dåvarande röststyrningar i mobiltelefonerna skapade hög distraktionsgrad hos föraren.

===Pollenallergi===
En fjärdedel av befolkningen har någon form av pollenallergi<ref>[http://www.expressen.se/halsa/paskhelgen-kommer-med-elaka-bjorkpollen/ Expressen 2014-04-16: Påskhelgen kommer med elaka björkpollen]</ref><ref>[http://pollenkoll.se/om-pollen Pollenkoll.se: Om pollen]</ref>. Pollensäsongen pågår i flera perioder: På våren är det lövträden, under sommaren är det gräset och på sensommaren/hösten så blommar gråbon. De flesta allergiker lindrar sina symtom med receptfria antihistaminer. Och här kommer "problemet". Antihistaminer har nämligen visat sig öka risken för trafikolyckor.

En metaanalys från 2004 sponsrad av amerikanska "National Highway Traffic Safety Administration" visar att riskerna med antihistaminer är högst för den första generationen antihistaminläkemedel som hade starkare sederande effekt (beroende på affinitet till bl.a muskarinreceptorer), men det finns även flera nedsättningar kopplade till den andra generationens läkemedel. Ett anmärkningsvärt exempel är cetrizin, ett populärt, receptfritt läkemedel som många använder. Det uppvisar samma typ av nedsättningar som cannabis. Forskarna noterar att studierna som upptäckte högre risker med cetrizin såg att den negativa effekten uppstod en längre tid efter doseringen än vad många andra studier innefattade i sitt mätområde, vilket kan indikera att den verkliga risken är undervärderad:

{{quote|The single largest contributing factor in fatal motor vehicle crashes in the United States is alcohol-induced impairment (AMA Council on Scientific Affairs, 1986). While this has been the case for many years, there also has been an increasing awareness of the traffic safety risks due to the behavioral toxicity of drugs other than alcohol. These include not only illicit drugs, such as cocaine and marijuana, but also medicinal drugs available by prescription or over the counter. In particular, the widespread use of antihistamines (i.e., histamine H1-receptor antagonists, or H1-antagonists for short) presents a particular focus for concern since the 1st-generation H1-antagonists are well recognized for often causing sedation and central nervous system (CNS) dysfunction which can jeopardize safe driving. Moreover, these drugs also have additive effects with alcohol and other CNS depressants.

...

'''Driving And Piloting'''

...this category includes measures of actual driving on the road, or in a closed course, as well as a variety of measures from many different types of driving simulators and some piloting tasks. ... significant impairment of any type of driving-related behavior was found in 29% (2 of 7 tests) of 
the findings for cetirizine and in 13% (2 of 16 test findings) for terfenadine.

'''Tracking:'''

This behavioral category included measures of different types of tracking tasks, including pursuit, compensatory, critical and adaptive tracking. Significant impairment was 
reported for 69% (33 of 48 tests) versus 19% (6 of 32 tests), respectively, of the findings for the 1st and 2nd generation drugs. ...all five of the 1st generation drugs demonstrated significant impairment for nearly all test findings reviewed. In contrast, for the five 2nd-generation drugs tested, only cetirizine and fexofenadine were found to impair tracking. Specifically, two of the three findings for cetirizine, and both of the two findings for fexofenadine, showed significantly impaired tracking performance.

'''Reaction Time:'''

This category included simple and complex reaction time tasks,... For the 1st generation drugs, 48% (29 of 61) of the test findings were found to show significant slowing of reaction time; this compares to 11% (4 of 37 findings) for the 2nd generation drugs. ...diphenhydramine and tripolidine, respectively, had the most notable impairing effects 
(54% or 13 of 24 findings, and 50% or 6 of 12 findings), whereas cetirizine was the only 2nd generation drug showing significant impairment (40%, 4 of 10 findings). 

...

Most studies tested for impairment or sedation within the window of expected peak drug effects, typically at two to three hours post-dose. Some studies utilized repeated test batteries over a five to eight hour period. However, in certain cases the lack of significant findings appeared due to testing either too early, or too late, to capture the peak drug effects. For example, two of the significant findings of impairment by cetirizine only occurred on specific measures and at much later times in the testing session, namely between 6 and 8 hours post-dose (Gengo et al., 1990; Walsh et al., 1992). Such effects clearly would be missed if the testing had only included a more limited number of measures or only earlier post-dose times as many of the other studies had done.|Moskowitz & Wilkinson, (2004)<ref>[http://www.nhtsa.gov/people/injury/research/antihistamines/antihistamines%20web/images/antihistamines%20text.pdf Antihistamines And Driving-Related Behavior: A Review Of The Evidence For Impairment (Moskowitz & Wilkinson, 2004)]</ref>}}

Hur allvarligt är då detta?

En studie från 1992 angav en riskratio (RR) på 1.2 (95% konfidensintervall 0.6 to 2.4)<ref>[http://www.ncbi.nlm.nih.gov/pubmed/1442753 Psychoactive drugs and the risk of injurious motor vehicle crashes in elderly drivers (Ray, 1992)]</ref> och andra studier ligger runt samma siffra. Riskökningen är inte alarmerande och meningen med detta stycket är inte att blåsa upp faran. Det intressanta är att större delen av alla bilförare utsätter sig för olika riskökningar utan att bli föremål för skräckpropaganda och förbud. En metaanalys från 2013<ref name=elvik></ref> uppskattar antihistaminers oddsratio till 1.12, en siffra som i samma analys är likvärdig med oddsration för cannabis. Det borde egentligen inte förekomma mer skräckpropaganda om cannabis i trafiken än för antihistaminer.

==Se även==
[[Cannabismetaboliternas psykoaktivitet]] går in djupare på blodkoncentration av THC och dess metaboliter och utvärderar vid vilken nivå som negativa effekter inte längre kan påvisas. 

[http://norml.org/library/item/cannabis-and-driving-a-scientific-and-rational-review Norml: Cannabis and Driving: A Scientific and Rational Review]

[http://www.nhtsa.gov/people/injury/research/stateofknwlegedrugs/stateofknwlegedrugs/pages/TOC.html State of Knowledge of Drug-Impaired Driving (Jones, 2003)]

I flera av studierna vi har refererat till i detta kapitlet så antyder man att den cannabispåverkade kan kompensera för drogens negativa sidor. I ett videoklipp från BBC-serien "Fifth Gear" så tar programledaren Quentin Willson och kör runt med en ung man på en hinderbana, personen får sedan röka cannabis och köra samma runda igen. Hans egna kommentarer under den andra körningen är att han känner sig mer paranoid för att göra något fel och känner att han måste lägga mer fokus på körningen än innan. Detta är kompenseringen. Resultatet blir t.o.m bättre under andra körningen då han slarvade och körde på en vägkon när han var nykter.

'''Fifth Gear S01E04 - Reefer Madness'''{{#ev:youtube|l5C0xdwbg_4}}


En annan dokumentär visar vad som händer när en icke-rökande kvinna röker cannabis och sätter sig i bilen för att göra några tester på en bana, hon gör även samma test med alkohol. Med cannabis kan man se att hon övervärderar farorna och blir ytterst rädd och försiktig, kör väldigt sakta och tvärnitar långt innan faran. Med alkohol blir hon istället rallyföraren som tror hon är en jättebra förare, kör för snabbt och kör sönder allt i sin väg. Inslaget börjar 32 minuter in i dokumentären. Det intressanta är att detta är en dokumentär med starka anti-cannabis-åsikter.

'''Should I Smoke Dope?'''{{#ev:youtube|uHGsgDLXCns}}


En annan TV-kanal filmade ett experiment där tre personer (en medicinsk marijuana-patient och två personer med låg konsumtion) fick röka stora mängder cannabis och köra runt i en hinderbana. Personen som rökte mest var cannabispatienten. Hon hade samtidigt inga större problem att ta sig runt banan, varken första gången då hon kom till testet och redan hade nivåer 3 gånger över vad som är olagligt i Washington eller när hon rökt ännu mera. En av konerna som åkte i backen när hon rundade ett hörn träffades av kameran som var monterad en bit ut från bilens sida. Från de andra personerna kunde man även se ett exempel på extremt låga hastigheter och att personerna inte tyckte att de kände sig bekväma bakom ratten, vilket åter igen pekar på att användaren av cannabis är medveten om sin nedsatta förmåga, till skillnad från den vanliga effekten av alkohol där risktagandet och förtroendet för den egna förmågan istället ökar.

'''How high is too high? KIRO tests pot-smoking drivers to find out'''{{#ev:youtube|d6gLrnJxS8c}}

{{legaliseringsguiden}}