People who drive after using cannabis are twice as likely to be involved in a crash resulting in serious injury or death than those who drive while unimpaired, according to a meta-analysis in the BMJ.

In order to answer the question ‘does the acute consumption of cannabis increase the risk of a motor vehicle collision?’ the authors undertook a systematic review and meta-analysis of observational studies. Nine case-controlled studies were included. All the studies focused on collisions resulting in serious injuries and deaths, rather than minor collisions.

The authors found significant heterogeneity across the studies.

The overall finding was that driving under the influence of cannabis was associated with a significantly increased risk of motor vehicle collisions, with an odds ratio (OR) 1.92 (95% CI: 1.35-2.73, p = 0.0003). The increased risk was higher in studies of fatal collisions OR 2.10 (95% CI: 1.31-3.36, p = 0.002).

There was insufficient data on tetrahydrocannabinol concentrations for the authors to be able to comment on dose-response effects.

Despite the high prevalence of cannabis use and its intoxicating effects, there is very little published information on the real life risks of driving when having used cannabis. ‘These results converge with those from experimental studies (laboratory, simulator, and forensic) suggesting that cannabis impairs performance of the cognitive and motor tasks necessary for safe driving, increasing the risk of collision,’ the authors conclude.

It seems obvious that driving under the influence of any sedating drug is potentially dangerous, but GPs can now advise their cannabis-using patients that they are twice as likely to be involved in a serious, potentially fatal, accident if they drive while ‘stoned’.

Dr Jez Thompson

• Asbridge M, Hayden JA, Cartwright JL. Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis. BMJ 2012;344:e536