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Acute Cannabis Consumption and Motor Vehicle Collision Risk: Systematic Review of Observational Studies and Meta-Analysis


Acute Cannabis Consumption and Motor Vehicle Collision Risk: Systematic Review of Observational Studies and Meta-Analysis

Mark Asbridge et al. BMJ.


Objective: To determine whether the acute consumption of cannabis (cannabinoids) by drivers increases the risk of a motor vehicle collision.

Design: Systematic review of observational studies, with meta-analysis.

Data sources: We did electronic searches in 19 databases, unrestricted by year or language of publication. We also did manual searches of reference lists, conducted a search for unpublished studies, and reviewed the personal libraries of the research team. Review methods We included observational epidemiology studies of motor vehicle collisions with an appropriate control group, and selected studies that measured recent cannabis use in drivers by toxicological analysis of whole blood or self report. We excluded experimental or simulator studies. Two independent reviewers assessed risk of bias in each selected study, with consensus, using the Newcastle-Ottawa scale. Risk estimates were combined using random effects models.

Results: We selected nine studies in the review and meta-analysis. 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); we noted heterogeneity among the individual study effects (I(2)=81). 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).

Conclusions: 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.

Conflict of interest statement

Competing interests: All authors have completed the Unified Competing Interest form at (available on request from the corresponding author) and declare: all authors had financial support from Dalhousie University and the Canadian Institutes of Health Research for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.


Fig 1 Study selection for systematic review
Fig 2 Meta-analysis of observational studies investigating the association between acute cannabis consumption and motor vehicle crashes
Fig 3 Pooled odds ratio (95% CI) of motor vehicle collision risk with tetrahydracannabinol for subgroups of studies
Fig 4 Funnel plot examining possible publication bias in studies investigating the association between acute cannabis consumption and risk of motor vehicle crashes. The horizontal axis measures the association (odds ratio) observed in individual studies, plotted against the standard error. The broken vertical line indicates our overall pooled estimate from meta-analysis (odds ratio 1.92)

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