By Anne Harding

(Reuters Health) – Heavy marijuana users who picked up the habit before age 16 have impaired driving skills even when they’re not high, a small study suggests.

But chronic cannabis consumption starting later in life was not associated with worse performance behind the wheel, Dr. Staci A. Gruber, director of the Marijuana Investigations for Neuroscientific Discovery (MIND) Program at McLean Hospital in Belmont, Massachusetts, and her colleagues found.

“This is not surprising to us at all, and really just underscores what we’ve already seen,” Gruber told Reuters Health in a phone interview. “Earlier onset of the use of any substance is more likely to confer a greater risk of problems later.”

Eleven U.S. states and Washington, DC, have legalized adult marijuana use, while 33 states have medical marijuana programs, Gruber and her team note in their report in the journal Drug and Alcohol Dependence. Research on cannabis and driving has focused on acute intoxication, with some studies but not others showing that people drive more slowly when high. Studies looking at cannabis intoxication and car crashes have also had mixed results.

To investigate whether heavy cannabis use might have residual effects on driving performance, the authors had 17 non-cannabis-using healthy participants and 28 chronic cannabis users complete a 10-minute, 4.2-mile driving simulation test. The cannabis users were instructed to abstain for at least 12 hours beforehand.

Participants ranged in age between about 18 and 28 years old. Half of the cannabis-using group had begun heavy use before age 16.

The early-use group had more crashes, missed more stop signs and red lights, and spent more time driving over the speed limit than the control group. However, there were no differences in any measure of driving performance between the participants who started using cannabis when they were older and the control group of non-users.

In fact, the analysis found that self-reported impulsivity accounted for almost all of the difference in results between the early-use and control groups.

“Early exposure to cannabis results in difficulties with different types of cognitive tasks. It really does come down to a question of age of onset of exposure,” Gruber said. “Individuals who are using cannabis on a regular, consistent basis, heavy cannabis users, prior to age 16 appear to look different from those who start using heavy amounts of cannabis after age 16.”

The study wasn’t designed to determine whether or how early cannabis use might influence impulsivity or the reverse, nor can it say how either of these traits may affect driving skills later in life.

While pushing adolescents to abstain from cannabis and other substances isn’t effective, Gruber noted, recommending that they wait could be a more useful approach. “It’s not a message of ‘just say no,’ it’s a message of ‘just not yet,'” Gruber said. “We’re not saying never, we’re saying right now you’re in a period of vulnerability, why not give yourself a little more time.”

The study “raises almost more questions than it answers,” Dr. Thomas G. Brown, an addiction researcher at the Douglas Mental Health University Institute and McGill University in Montreal, told Reuters Health in a phone interview.

Adolescents who are prone to risky behavior may be more likely to become heavy pot smokers, and these personality traits may also make them riskier drivers, explained Brown, who wasn’t involved in the study. “We don’t know from this study whether what we’re seeing is cannabis use, or the consequence of these personality features that promote risk taking of all sorts.”

SOURCE: Drug and Alcohol Dependence, online January 14, 2020.