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

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.