A meta-analysis of nine epidemiological studies found that drivers who tested positive for marijuana or self-reported using marijuana were more than twice as likely as other drivers to be involved in motor vehicle crashes. Guohua Li, professor of epidemiology and anesthesiology, and his colleagues looked at nine epidemiologic studies published in English over the past two decades. Although the increased risk was consistent across geographic regions and driver populations, the authors caution that it is impossible to infer causality from epidemiologic data alone. Complicating factors include multiple-drug use and varying assessments of marijuana use across studies.
The study appeared online on October 4, 2011, in Epidemiologic Reviews.