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

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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.


“This doesn’t mean if you put heavy cannabis users on the road they’re going to mow people down and drive horribly — it doesn’t mean they’d hit impaired levels of driving in the real world,” Gruber said in an interview. “It just means there are differences.”

To study whether heavy cannabis use could degrade driving performance even when users weren’t high, the McLean team recruited 45 healthy subjects to complete a 4.2-mile virtual course in a driving simulator. Of them, 17 did not use marijuana, 14 consumed the drug heavily and began doing so before age 16, and 14 started later in life. The course included stop signs, traffic lights, yields, merges, pedestrians, and other vehicles; participants were graded on how well they followed traffic rules, stayed in their lanes, and reacted to unexpected hazards.

The study defined heavy users as those who had consumed marijuana five out of the previous seven days, reported at least 1,500 lifetime uses of the drug, and tested positive for cannabanoids.

The peer-reviewed paper, which will be published in the journal Drug and Alcohol Dependence, called for further research, speculating that similar differences in driving performance may also be present among chronic users of other substances such as alcohol and prescription medication. Its authors also acknowledged the small sample size of their own study, but they said their subjects were unusually well-screened for other variables that could affect driving performance and that the highly significant results could likely be replicated in larger groups.


According to a 2019 summary prepared by the Congressional Research Service, previous studies have been unable to consistently find a link between marijuana use and real-world car crashes.

“Although laboratory studies have shown that marijuana consumption can affect a person’s response times and motor performance, studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage,” the congressional report concluded.

In fact, the report added, scientists have struggled even to find a correlation between a users’ level of impairment and the amount of THC in their bloodstreams, with the same quantities of the drug apparently having substantially different effects on different users. Furthermore, existing blood, urine, and hair tests can only show someone used marijuana in recent days or weeks — not whether they were impaired at the moment the test was administered.

Citing that body of work, Gruber urged policymakers not to use her team’s new study as a justification for “per se” limits. These are legal standards adopted by some states under which suspected stoned drivers whose blood or urine contains a certain level of marijuana metabolites are automatically presumed to be impaired.


While her new study shows a subset of heavy marijuana users may drive differently from non-users even when sober, Gruber said, that’s hardly a reason to punish everyone whose system contains traces of the drug — especially since forthcoming McLean data suggest the drug may actually help some medical marijuana patients perform better cognitively.

In Massachusetts, Governor Charlier Baker has proposed a bill that would crack down on stoned driving in part by forcing courts to accept testimony from police trained as so-called “drug recognition experts.” While Baker’s measure would not establish per se limits, tests administered by such officers include a biological test for marijuana (although these tests cannot determine if someone was stoned at that particular moment). If the bill passes, drivers who refuse to take the test would have their licences revoked for six months; it’s facing opposition from civil rights groups.

Other cannabis researchers praised the McLean study, saying that while its small sample size means further inquiry is needed, the results align with a growing scientific consensus that heavy marijuana use — especially in adolescence — can have significant negative effects beyond the immediate high.

“The implications of this are hugely important,” said Dr. Randi Schuster, the director of neuropsychology at the Center for Addiction Medicine at Massachusetts General Hospital and a prominent researcher of adolescent cannabis use. “It really speaks to a sensitive time period during development when, if use is initiated, the consequences might be greater. And it confirms what we’re seeing elsewhere in cannabis, which is that even when somebody is not acutely intoxicated, they’re still showing residual impairment in learning, attention, and other cognitive domains.”


Schuster said that even though driving drunk is far riskier, driving high is still dangerous — yet many adolescents she works with have a blasé attitude about getting behind the wheel when stoned. The McLean study should serve as a warning to parents and teens alike, she said.

“From a consumer perspective, it’s important to know you might still be impacted even when not actively high,” Schuster said.

Dan Adams can be reached at daniel.adams@globe.com. Follow him on Twitter @Dan_Adams86.