A study that appeared in the December 2005 British Medical Journal, evaluated the relative risk of being responsible for a fatal crash while driving under the influence of cannabis, the prevalence of such drivers within the driving population, and the corresponding share of fatal crashes.

The study looked at 10,748 drivers, with known drug and alcohol concentrations, involved in fatal crashes in France from October 2001 to September 2003. 681 drivers were positive for cannabis (cases 8.8%, controls 2.8%), including 285 with an illegal blood alcohol concentration (0.5 g/l).

The presence of cannabis was associated with increased risk of responsibility (odds ratio 3.32). A significant dose effect was identified; the odds ratio increased from 2.18 with a low level of 9tetrahydrocannabinol (<1 ng/ml) to 4.72 at a higher level ( 5 ng/ml). The effect of cannabis remains significant after adjustment for different cofactors, including alcohol, with which no statistical interaction was observed.

The prevalence of cannabis (2.9%) estimated for the driving population is similar to that for alcohol (2.7%). At least 2.5% of fatal crashes were estimated as being attributable to cannabis, compared with at least 28.6% for alcohol.

The study concluded that driving under the influence of cannabis increases the risk of involvement in a crash. However, in France its share in fatal crashes is significantly lower than that associated with positive blood alcohol concentration.

The study was authored by Bernard Laumon, Blandine Gadegbeku, Jean-Louis Martin and Marie-Berthe Biecheler. 

Reprinted from the Winter 2006 issue of the Reporter, the newsletter of the International Council on Alcohol, Drugs and Traffic Safety.