Researchers have found that marijuana use may have an impact on driving ability even 12 hours after use.
The study, published in the journal Drug and Alcohol Dependence, found that in addition to chronic, heavy, recreational cannabis use being associated with poorer driving performance in non-intoxicated individuals compared to non-users.
While several studies have examined the direct effect of cannabis intoxication on driving, no other studies until now have examined the effects on driving in heavy marijuana users who are not high.
"People who use cannabis don't necessarily assume that they may drive differently, even when they're not high," said study researcher Staci Gruber from McLean Hospital in the US.
"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," Gruber added.
For the findings, the research team used a customised driving simulator to assess the potential impact of cannabis use on driving performance.
At the time of study, marijuana users had not used for at least 12 hours and were not intoxicated.
Overall, heavy marijuana users demonstrated poorer driving performance as compared to non-users.
For example, in the simulated driving exercise, marijuana users hit more pedestrians, exceeded the speed limit more often, made fewer stops at red lights, and made more center line crossings.
When researchers divided the marijuana users into groups based on when they started using cannabis, they found that significant driving impairment was detected and completely localized to those who began using marijuana regularly before age 16.
"It didn't surprise us that performance differences on the driving simulator were primarily seen in the early onset group," said study researcher Mary Kathryn Dahlgren.
According to the authors, research has consistently shown that early substance use, including the use of cannabis, is associated with poorer cognitive performance.
"What was interesting was when we examined impulsivity in our analyses, most of the differences we saw between cannabis users and healthy controls went away, suggesting that impulsivity may play a role in performance differences," Dahlgre added.
"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," she said. o "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," she added.