Cannabis consumption doubles the risk of serious motor vehicle accidents

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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

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Andre Tomlin

Andre Tomlin

André Tomlin is an Information Scientist with 20 years experience working in evidence-based healthcare. He's worked in the NHS, for Oxford University and since 2002 as Managing Director of Minervation Ltd, a consultancy company who do clever digital stuff for charities, universities and the public sector. Most recently André has been the driving force behind the Mental Elf and the National Elf Service; an innovative digital platform that helps professionals keep up to date with simple, clear and engaging summaries of evidence-based research. André is a Trustee at the Centre for Mental Health and an Honorary Research Fellow at University College London Division of Psychiatry. He lives in Bristol with his wife, dog and three little elflings.

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