Cannabis (marijuana) is the most widely used illicit drug in the world and reports of driving under the influence of cannabis have risen in recent years.
A research team from Dalhousie University in Canada conducted a systematic search and identified observational epidemiology studies of motor vehicle collisions with an appropriate control group. They included studies that measured recent cannabis use in drivers by toxicological analysis of whole blood or self report.
Nine studies were included in their review and meta-analyses and the reviewers noted heterogeneity among the individual study effects (I2=81).
The full-text of the review is available for free in the BMJ (see link below).
Here’s what they found:
- 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).
- 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).
The authors concluded:
Acute cannabis consumption is associated with an increased risk of a motor vehicle crash, especially for fatal collisions. This information could be used as the basis for campaigns against drug impaired driving, developing regional or national policies to control acute drug use while driving, and raising public awareness.
The accompanying editorial noted that:
Public health education about the dangers of driving while under the influence of cannabis is unlikely to be enough to deter cannabis users from driving.
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