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"Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded."

Abraham Lincoln - Former U.S. President

Cannabis & Health

Cannabis & Driving

Cannabis & Driving

by Dr Greg Chesher [ This email address is being protected from spambots. You need JavaScript enabled to view it. ]


I want to mention the present understanding about Cannabis and driving. You may already be quite familiar with these studies, but in case you are not, the following might interest you. The law about alcohol which proscribes driving with BAC >0.05 g% blood alcohol has been determined by epidemiological studies employing the case-control method. At 0.05g% the driver is about twice as likely to be involved in a crash as a driver with zero BAC. As the BAC increases, so in a very steep fashion, does the probability of being involved in a crash. The role of alcohol in road crashes has been very clearly demonstrated. The drink-driving laws are based upon sound science!

As for other drugs, including Cannabis, the pharmacokintetics do not permit this technique. There is no correlation between the blood concentration of Cannabinoids and impairment as determined on laboratory tasks. Furthermore, the study would require the taking of blood from the crash involved drivers as well as that of the control drivers--not involved in a crash. There is no equivalent to the alcohol breathalyser. Cannabinoids are not excreted on the breath. (It is the collection of adequate data from a control group that really precludes the case-control method). So to overcome this, the technique of "culpability analysis" has been employed.

In this case, data for crashes are presented to an independent group of observers who are ignorant (blind) as to whether any of the drivers had any drug at all in their blood. By studying the information of the crash (events before and up to the crash) a score apportioning the degree of blame to each driver is given. From this "culpability" is determined.

So far there have been four studies using this technique, two in the USA and two in Australia.

As far as Cannabis is concerned, it is quite remarkable that in all cases the results were so similar as to be the same finding. The culpability ratio for the drivers bearing Cannabinoids in blood revealed that they were no more likely to be considered as a cause of the crash as those who had no drugs at all in their blood. Indeed in each of these four studies the Cannabinoid drivers were less (but not significantly so) likely to have been a cause than those with no drug at all. In all of these studies alcohol-bearing drivers were overwhelmingly culpable.

So at the moment, the present data suggest that cannabis is not involved as a causative factor in road crashes.

Two things must be considered. First, it is early days so far. The total number of cases in all of these studies totals about 9000 or a little less.

Second, the determination of the culpability score is dependent upon the integrity of the initial data describing the accident. In most cases this is from the attending police. This is not always reliable. However, as the numbers increase with more studies, we will have more confidence with the findings.

There is another Australian study currently being analysed and is due to be released shortly.....SOON is the advice I have from the author.


Greg Chesher



Drummer, O. (1994). Drugs and drivers killed in Australian Road traffic accidents. The use of responsibility analysis to investigate the contribution of drugs to fatal accidents: Victorian Institute of Forensic Pathology. Monash University.

Hunter, C., Lokan, R., Longo, M., White, J. & White, M. (1998). The Prevalence and Role of Alcohol, Cannabinoids, Benzodiazepines and stimulants in Non-Fatal Crashes. Adelaide: Forensic Science, Department for Administrative and Information Services, South Australia.

Terhune, K., Ippolito, C., Hendricks, D., Michalovic, J., Bogema, S., Santinga, P., Blomberg, R. & Preusser, D. (1992). The incidence and role of drugs in fatally injured drivers.: US Department of Transportation National Highway Traffic Safety Administration.

Williams, A., Peat, M., Crouch, D., Wells, J. & Finkle, B. (1985). Drugs in fatally injured young male drivers. Public Health Reports, 100, 19-25