Many young people know not to drive while drunk but in the wave of health claims and legislative endorsements of marijuana, the message is being lost that you will still be impaired if you are high.

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 - 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 says 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 the 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