Driving under the influence of cannabis almost doubles the risk of a fatal road crash, finds a study published online by the BMJ today. However its share in fatal crashes is significantly lower than those involving alcohol.

The study took place in France and involved 10,748 drivers who were involved in fatal crashes from October 2001 to September 2003. All drivers underwent compulsory tests for drugs and alcohol.

A total of 681 drivers tested positive for cannabis (7%) and 2096 for alcohol (21.4%), including 285 for both (2.9%). Men were more often involved in crashes than women, and were also more often positive for both cannabis and alcohol, as were the youngest drivers, and users of mopeds and motorcycles.

The risk of being responsible for a fatal crash increased as the blood concentration of cannabis increased (known as a dose effect). The odds increased from 1.9 at a concentration of 0-1 ng/ml to 3.1 at or above 5 ng/ml. These effects were adjusted for alcohol and remained significant when also adjusted for other factors.

These results give credence to a causal relationship between cannabis and crashes, say the authors.

Samples show that the prevalence of cannabis (2.9%) within the driving population is similar to that for alcohol (2.7%) at or above 0.5 g/l, they add. However, in France, its share in fatal crashes is significantly lower than that associated with alcohol (2.5% compared with 29% for alcohol).