Study: Early-onset Recreational Marijuana Use Linked with Impaired Driving Ability When Sober

Recreational marijuana use affects driving ability even when users are not high on marijuana, according to the findings of a study published in the journal Drug and Alcohol Dependence.

To conduct this study, researchers from McLean Hospital (psychiatric affiliate of Harvard Medical School) used a customized driving simulator to appraise the potential impact of marijuana use on driving performance. At baseline, all marijuana users had not used for at least 12 hours and were not intoxicated.

The results showed that marijuana use has a detrimental affect on driving as users demonstrated poorer driving performance vs. non-users. In the simulation, marijuana users hit more pedestrians, exceeded the speed limit more often, made fewer stops at red lights, and made more center line crossings. The researchers concluded that earlier onset marijuana use, defined as regular use before the age of 16, was associated with poorer driving performance.

“It didn’t surprise us that performance differences on the driving simulator were primarily seen in the early onset group,” said one of the study’s lead researchers, Mary Kathryn Dahlgren, PhD, in a press release. “Research has consistently shown that early substance use, including the use of cannabis, is associated with poorer cognitive performance.”

“People who use cannabis don’t necessarily assume that they may drive differently, even when they’re not high,” explained the other lead researcher, Staci Gruber, PhD. “We’re not suggesting that everyone who uses cannabis will demonstrate impaired driving, but it’s interesting that in a sample of non-intoxicated participants, there are still differences in those who use cannabis relative to those who don’t.”

“There’s been a lot of interest in how we can more readily and accurately identify cannabis intoxication at the roadside, but the truth of the matter is that it is critical to assess impairment, regardless of the source or cause,” Dr. Gruber added.

“It’s important to be mindful that whether someone is acutely intoxicated, or a heavy recreational cannabis user who’s not intoxicated, there may be an impact on driving, but certainly not everyone demonstrates impairment simply as a function of exposure to cannabis. This is especially important to keep in mind given increasing numbers of medical cannabis patients who differ from recreational users with regard to product choice and goal of use.”