Background About one third of fatally injured drivers in the United States test positive for non-alcohol drugs and 20% test positive for two or more drugs. Alcohol-marijuana is the most commonly detected poly-drug combination. The role of concurrent use of alcohol and marijuana in crash causation has not been well understood.

Methods Using a case-control design, we assessed the individual and joint effects of alcohol and marijuana on fatal crash risk. Cases (n = 2613) were drivers who were involved in fatal motor vehicle crashes in the continental United States during specific time periods in 2006, 2007, and 2008 and controls (n = 7719) were participants of the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers.

Results Overall, 53.3% of the cases and 8.9% of the controls had elevated blood alcohol concentrations (BAC ≥ 0.01 g/dL), and 6.0% of the cases and 3.0% of the controls tested positive for marijuana. Compared to drivers testing negative for both alcohol and marijuana, the estimated odds ratios of fatal crash involvement were 11.77 [95% confidence interval (CI): 10.78, 12.84] for those testing positive for alcohol and negative for marijuana, 2.16 (95% CI: 1.85, 2.54) for those testing negative for alcohol and positive for marijuana, and 25.46 (95% CI: 21.25, 30.51) for those testing positive for both alcohol and marijuana.

Conclusions Alcohol and marijuana are each associated with a significantly increased risk of fatal crash involvement. When alcohol and marijuana are used together, there exists a positive interaction effect on the risk of fatal crash involvement on the additive and the multiplicative scales.