Heavy marijuana users who started young drive differently — even while not stoned, study says

Boston Globe 14 January 2020
Family First Comment: “People who started using marijuana heavily before they were 16 exhibit poorer driving performance than those who abstain — even when they’re not high, according to a new study that underlines the risks of adolescent cannabis consumption.”

People who started using marijuana heavily before they were 16 exhibit poorer driving performance than those who abstain — even when they’re not high, according to a new study that underlines the risks of adolescent cannabis consumption.

The research, conducted by a team at McLean Hospital in Belmont and set to be published this month, is the first to find a link between marijuana use and diminished driving performance when the user isn’t actively stoned. Most previous studies have focused on the effects of acute impairment, testing how people drove shortly after consuming pot.

The study of 45 subjects found heavy cannabis users using a driving simulator hit more pedestrians, missed more stop signs and red lights, drove faster, and left their lane more often than non-users, even after abstaining from the drug for at least 12 hours. But those differences became insignificant when researchers removed from the sample those who began using pot heavily before 16, suggesting the effect is almost entirely limited to that group. Heavy consumers who started later in life drove about as well as those who abstain.

The McLean researchers, led by Drs. Mary Kathryn Dahlgren and Staci Gruber, cautioned their paper does not prove heavy, early users are functionally impaired behind the wheel on real-world roads. Instead, they said, the work echoes earlier findings by their lab that heavy marijuana use during critical stages of adolescent brain development is associated with poorer cognitive performance, including at some of the various mental tasks required to drive.

“What we’re seeing is relatively poor performance in early users compared to our non-using controls, but not necessarily impairment,” Dahlgren said. “We don’t want to make any firm statements about causality.”

A complex web of potentially confounding factors make it difficult to directly tie heavy marijuana use to impaired driving in the real world, the researchers said. One of the most significant is impulsivity, a trait that was generally stronger among the heavy cannabis users — though it’s unclear whether impulsivity helps cause heavy use or is a result of it (or both). Impulsivity on its own may also contribute to poor driving, and when controlled for in the McLean driving simulator study, erased most of the performance differences between all the heavier cannabis users and the non-users.
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