Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysisBMJ 2012; 344 doi: https://doi.org/10.1136/bmj.e536 (Published 09 February 2012) Cite this as: BMJ 2012;344:e536
- Mark Asbridge, associate professor,
- Jill A Hayden, assistant professor,
- Jennifer L Cartwright, research coordinator
- 1Department of Community Health and Epidemiology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 1V7
- Correspondence to: M Asbridge
- Accepted 12 December 2011
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 We also did manual searches of reference lists, conducted a search for unpublished studies, and reviewed the personal libraries of the research team.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 (I2=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.
Contributors: MA and JAH designed data collection tools, monitored data collection, and drafted and revised the paper. MA and JLC assessed articles for inclusion in the review and for risk of bias and study quality, with JAH as consulting author in case of disagreements. JLC collected and analysed the data, and revised the paper. The review was carried out with the assistance of two decision makers; the Nova Scotia Road Safety Advisory Committee and the Canadian Centre on Substance Abuse. All authors had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Funding: This review was funded by the Canadian Institutes of Health Research. The funding source had no role in study design; in the collection, analysis, and interpretation of data; in the writing of the report; or in the decision to submit this article for publication. All researchers worked independently of the funding agency.
Competing interests: All authors have completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (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.
Ethics approval: Not required.
Data sharing: Search strategies and results, as well as details of excluded studies at each stage of the search are available on request from the corresponding author.
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