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

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.