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. 2017 Dec;4(1):8.
doi: 10.1186/s40621-017-0105-z. Epub 2017 Mar 20.

Interaction of marijuana and alcohol on fatal motor vehicle crash risk: a case-control study

Stanford Chihuri  1   2 , Guohua Li  3   4   5 , Qixuan Chen  6
Affiliations

Interaction of marijuana and alcohol on fatal motor vehicle crash risk: a case-control study

Stanford Chihuri et al. Inj Epidemiol. 2017 Dec.

Abstract

Background: Concurrent use of marijuana and alcohol in drivers is of increasing concern but its role in crash causation has not been well understood.

Methods: Using a case-control design, we assessed the individual and joint effects of marijuana and alcohol use on fatal crash risk. Cases (n = 1944) were drivers fatally injured in motor vehicle crashes in the United States at specific times in 2006, 2007 and 2008. Controls (n = 7719) were drivers who participated in the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers.

Results: Overall, cases were significantly more likely than controls to test positive for marijuana (12.2% vs. 5.9%, p < 0.0001), alcohol (57.8% vs. 7.7%, p < 0.0001) and both marijuana and alcohol (8.9% vs. 0.8%, p < 0.0001). Compared to drivers testing negative for alcohol and marijuana, the adjusted odds ratios of fatal crash involvement were 16.33 [95% confidence interval (CI): 14.23, 18.75] for those testing positive for alcohol and negative for marijuana, 1.54 (95% CI: 1.16, 2.03) for those testing positive for marijuana and negative for alcohol, and 25.09 (95% CI: 17.97, 35.03) for those testing positive for both alcohol and marijuana.

Conclusions: Alcohol use and marijuana use are each associated with significantly increased risks of fatal crash involvement. When alcohol and marijuana are used together, there exists a positive synergistic effect on fatal crash risk on the additive scale.

Keywords: Accidents; Alcohol; Case–control studies; Drug interaction; Marijuana; Motor vehicles; Safety; Traffic.

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