Heavy marijuana users drive worse, even when not intoxicated
Recreational marijuana use can impact driving ability in heavy users even when they are not intoxicated, particularly in those who began using the drug before 16 years of age, according to a study published in Drug and Alcohol Dependence.
“People who use cannabis don’t necessarily assume that they may drive differently, even when they’re not high,” Staci A. Gruber, PhD, director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital, said in a press release. “We’re not suggesting that everyone who uses cannabis will demonstrate impaired driving, but it's interesting that in a sample of nonintoxicated participants, there are still differences in those who use cannabis relative to those who don’t.”
Gruber and colleagues used simulators to compare driving performance between cannabis users (n = 28) and healthy nonusers (n = 17). Cannabis users included in the study reported use in at least 5 of the past 7 days and 1,500 times in their lifetime and tested positive for urinary cannabinoids. However, they abstained from use for 12 hours prior to the study visit, so they were not intoxicated during assessment.
The driving simulation lasted approximately 10 minutes and included both urban and rural driving conditions. Participants also completed IQ tests and a self-reported assessment to measure their impulsive behaviors.
Compared with controls, cannabis users demonstrated significantly impaired driving. Cannabis users hit more pedestrians, failed to stop at more stop signs, stopped at fewer red lights, drove over the speed limit more often, and crossed over the center line more than nonusers.
When dividing cannabis users into groups by the age they started regularly using the drug, researchers found that those who began using cannabis consistently before they were 16 years of age were the primary cause of poor task performance in the cannabis users group.
Those who reported early-onset cannabis use had significantly more collisions, missed more stop signs, stopped less at red lights, and drove a greater percentage of distance over the speed limit. There was no significant difference in driving simulation variables between the control group and late-onset cannabis users.
“It didn't surprise us that performance differences on the driving simulator were primarily seen in the early onset group,” M. Kathryn Dahlgren, PhD, a research fellow in the Cognitive and Clinical Neuroimaging Core at McLean Hospital, said in a press release. “Research has consistently shown that early substance use, including the use of cannabis, is associated with poorer cognitive performance.”
When adjusting for participants’ self-reported impulsive behaviors, researchers found that the number of collisions and stops at red lights were the only significant differences between the groups, with early-onset users demonstrating worse driving skills.
This finding suggests that “impulsivity may play a role in performance differences,” Dahlgren explained. – by Erin Michael
Disclosures: The authors report no relevant financial disclosures.