AMHERST, Mass. – Underage male college students who report using marijuana in the month before they were surveyed had a high prevalence of driving under its influence and of riding with a marijuana-using driver, at a rate more than double that of driving or riding after alcohol use, say researchers from the University of Massachusetts Amherst School of Public Health and Health Sciences and University of Washington pediatrics department.

Among other things, this study found that among marijuana-using students, 44 percent of males and 9 percent of females drove after using the drug, and 51 percent of males and 35 percent of females rode as a passenger with a marijuana-using driver.

Lead author Jennifer Whitehill at UMass Amherst and colleagues say their findings probably reflect the widespread myth that driving after marijuana use is safe. The researchers suggest that developing strategies to combat this belief could help to change social norms and encourage using a designated driver not only after alcohol use, but after a driver has used any risky substance. Study findings are in the current issue of JAMA Pediatrics.

Whitehill says, “There seems to be a misconception that marijuana use is totally safe, but as an injury prevention researcher I dispute that. We’ve done a good job in public health with messages about the risks of driving after alcohol use. Clearly the idea not to drink and drive has come through for these students, because we found only 7 percent engage in that behavior. But our study suggests we must do better when it comes to marijuana, since we found that 31 percent of marijuana-using students drive under its influence.”

The health policy professor adds, “What I feel is, let’s create a culture where we don’t engage in any of these risk enhancing behaviors before we get behind the wheel.”

With the decriminalization of marijuana in 16 states and its availability increasing with medical use, concerns about drug-impaired drivers are more and more relevant, the authors point out. “The issue of marijuana-impaired driving is particularly salient for young drivers, for whom the combination of inexperience and substance use elevates crash risk. If they are part of a culture that accepts the behavior, their risks increase at a predictable rate that we understand better now,” Whitehill says.

The results describe the prevalence, sex differences and risk factors associated with underage college students’ driving after marijuana use, after using alcohol and riding with a driver using either substance.

Data for this report came from an ongoing longitudinal study. The researchers approached 640 incoming freshmen 18–20 years old at the University of Washington and University of Wisconsin, Madison, of who 338 agreed to take part. Overall, 30 percent of male and 13 percent of female study participants said they used marijuana in the 28 days prior to being surveyed, while 67 percent of the men and 64 of the women reported using alcohol over the same period. Overall, 23 percent of the men reported using both marijuana and alcohol over the previous month compared to just 9 percent of the women.

Whitehill notes, “Our findings are consistent with common knowledge and anecdotes indicating that students drive after using marijuana. And it wasn’t surprising to find that more men in particular drive after using marijuana compared to women. But our study quantifies the prevalence, which is useful in setting priorities for public health action. We also quantified the likelihood that someone will ride as a passenger with a driver who has used marijuana, and how much it rises with the proportion of their friends who use marijuana.”

Despite its limitations, including the small number of nonwhite participants and reliance on self-reported drug use, the study’s findings represent “an important and timely contribution” to knowledge about older adolescents’ driving after drug use, the authors say. Findings “supplement our knowledge that marijuana use increases the risk of motor vehicle crashes by estimating how common it is for underage students to have taken this risk with the past 28 days.”

This work was supported by grants from the NIH’s Common Fund, the National Institute of Drug Abuse and the National Institute of Child Health and Human Development.