WASHINGTON – Driving after marijuana use was common among college students at two large public universities who were randomly selected for phone interviews.
"Driving after marijuana use was significantly more common than driving under the influence of alcohol in college-age students, and males were much more likely ... which is consistent with a lot of the data on injury risk," reported Jennifer Whitehill, Ph.D., a postdoctoral fellow at the Harborview Injury Prevention and Research Center at the University of Washington, Seattle.
Among current marijuana users – defined as those who had used marijuana in the past 28 days – 31% said they drove after using it and 45% rode with a driver who had used it. The prevalence rates were 44% and 51%, respectively, among males and 9% and 35% among females, she reported at the annual meeting of the Pediatric Academic Societies.
The findings are especially important given new legislation that is likely to increase marijuana availability and further influence teens’ perception of risk.
The University of Washington, one of the two universities where the study took place, resides in one of two states that legalized the recreational use of marijuana in 2012. The study was conducted prior to passage of the law; continued research will provide "a good, natural longitudinal look" at attitudes and practices in light of the legislation, Dr. Whitehill said.
The literature thus far suggests a two- to threefold increase in crash risk for driving under the influence of marijuana, Dr. Whitehill said after the meeting.
One recent meta-analysis of information from nine prior studies showed that the risk of a crash was almost 2.7 times higher among marijuana users than nonusers (Epidemiol. Rev. 2012;34:65-72). Another review found an overall twofold increased risk, with risk estimates higher in studies of fatal collisions (BMJ 2012;344:e536).
However, the contribution of marijuana to the risk of crashes "can be a tricky thing to study and to isolate," she said, and research is attempting to determine the degree to which marijuana intoxication deteriorates driving performance.
Of 315 students who had just completed their freshman year, alcohol use was much more prevalent than marijuana use, as was expected – 65% said they had used alcohol in the past 28 days, compared with 21% who reported having used marijuana. Alcohol users were less likely to drive under the influence, however: 12% of males and 3% of females said they drove after drinking. More of the students – 21% of males and 12% of females – reported riding with a driver who had just consumed alcohol.
Researchers attempted to identify possible predictors and risk factors for impaired driving. They found that students who drove after marijuana use were 5.2 times as likely to have also been a passenger of a marijuana-using driver, and 2.5 times as likely to have also driven after alcohol use, as students who did not drive after using marijuana.
A younger age at the first use of marijuana was associated with a lower risk of driving under the influence of marijuana.
Students also were asked about their nonmedical use of stimulants and other prescription drugs. Four percent –a "relatively rare" prevalence – reported recent nonmedical prescription drug use, of which 11% said they drove after use, Dr. Whitehill reported.
The study did not measure the time between substance use and driving, she noted.
Dr. Whitehill reported that she had no relevant financial disclosures.