Long-term, heavy cannabis use might be making adults bad drivers. Especially, according to a new study in Drug and Alcohol Dependence, if drivers started consuming cannabis in their early teens. Researchers studying the impacts of recreational cannabis consumption on cognitive function say bad driving behaviors like speeding, ignoring traffic signals, and getting into accidents could stem from heavy adolescent cannabis use. Furthermore, researchers found that long-term cannabis users drove badly whether or not they were under the influence of THC.

Bad Driving is a Downstream Effect of Adolescent Cannabis Use, Study Says

Dr. Staci Gruber is an Associate Professor of Psychiatry at Harvard Medical School and the Director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital’s Brain Imaging Center. She’s an expert in the ways that substance use among adults and adolescents affects the brain, and her recent work looks at how cannabis affects cognitive ability and brain development. Her latest study, “Recreational cannabis use impairs driving performance in the absence of acute intoxication,” presents a new perspective on the relationship between cannabis and traffic safety.

No, this isn’t a study showing how being high makes you a bad driver. In fact, many studies that have looked into driving under the influence of THC have found that being high doesn’t have much of an impact on driving ability. In some cases, being high actually made drivers more cautious. Similarly, researchers have been unable to link expanding legal access to cannabis to any uptick in traffic accidents caused by drivers who were high behind the wheel.

Instead, Dr. Gruber’s new study is about how long-term heavy cannabis use impairs cognition, making complex cognitive tasks like driving more difficult.

The study’s findings, published Tuesday, resonate with other studies linking adolescent substance use to diminished cognitive performance later in life. Consuming mind-altering substances like alcohol and cannabis appear to disrupt the brain’s development at a crucial stage. Those disruptions lead to a range of cognitive and psychiatric problems down the line, not just bad driving.

“Prior to age 16, the brain is especially neurodevelopmentally vulnerable, not just to cannabis but to other drugs, alcohol, illness, injury,” said Gruber. “The brain is really under construction.”

Weed Can Make You a Bad Driver if You Consumed Heavily in Your Teens

To determine how adolescent cannabis use impaired driving ability later on in life, it was important for Dr. Gruber to assess driving performance in non-intoxicated cannabis users who consumed cannabis on a daily or near daily basis. For the purposes of the study, “heavy, long-term use” meant a minimum of 4 to 5 times a week, and with a lifetime exposure to cannabis of 1,500 times.

At first, the Dr. Gruber’s team found what they expected. Cannabis users demonstrated impaired driving compared with a group of drivers without a history of cannabis use. But they found something interesting when they broke down the results according to age group. According to the findings, significant driving impairment was detected and completely localized to those with early onset (before age 16) heavy cannabis use compared with the late onset group (after age 16).

So, the study concluded, chronic, heavy recreational cannabis use was associated with worse driving in non-intoxicated drivers. And the earlier someone became a chronic, heavy recreational cannabis user, the poorer they were at driving.

Study Highlights Cannabis’ Impact on Developing Minds

The study also found that impulsivity had a major impact on driving performance. And that’s exactly what researchers expected to see. “Research has consistently shown that early substance use, including the use of cannabis, is associated with poorer cognitive performance,” said study co-author Mary Kathryn Dahlgren. “Specifically tasks controlled by the most frontal part of the brain,” Dahlgren added.

The frontal lobe of the brain plays a major role in impulse response. It’s the part of the brain that significantly impacts how we make decisions. In this study’s findings, increased impulsivity overlapped with early-onset cannabis use, leading to poor decision-making on the road. And that, Gruber said, calls for further research.

We still don’t know whether impulsivity leads to adolescent cannabis use or vice versa. And the study’s authors also cautioned against taking its findings to suggest that cannabis consumers can’t drive. “By no means does this data suggest that everybody who uses cannabis is impaired and they can’t drive,” said Dahlgren.