Marijuana use may impair driving even when the high wears off

Recreational marijuana use can impair the driving ability of people even when they are sober, according to a study.

The study, published in the journal Drug and Alcohol Dependence, also linked earlier onset of marijuana use -- under age 16 -- to worse driving performance.

While previous research has examined the direct effect of cannabis intoxication on driving, no other studies until now have examined the effects on driving in heavy marijuana users who are not high.

The researchers from McLean Hospital in the US used a customised driving simulator to assess the potential impact of cannabis use on driving performance.

At the time of study, users had not smoked marijuana for at least 12 hours, and were not intoxicated.

Overall, heavy marijuana users demonstrated poorer driving performance as compared to non-users.

For example, in the simulated driving exercise, marijuana users hit more pedestrians, exceeded the speed limit more often, made fewer stops at red lights, and made more centre line crossings.

The researchers said the idea that differences can be detected in sober cannabis users may be surprising to the public.

"People who use cannabis don't necessarily assume that they may drive differently, even when they're not high," said Staci Gruber from McLean Hospital.

"We're not suggesting that everyone who uses cannabis will demonstrate impaired driving, but it's interesting that in a sample of non-intoxicated participants, there are still differences in those who use cannabis relative to those who don't," Gruber said.

The researchers divided the marijuana users into groups based on when they started using cannabis.

They found that significant driving impairment was detected, and completely localised to those who began using marijuana regularly before age 16.

"It didn't surprise us that performance differences on the driving simulator were primarily seen in the early onset group," McLean Hospital's Mary Kathryn Dahlgren said.

"Research has consistently shown that early substance use, including the use of cannabis, is associated with poorer cognitive performance," Dahlgren said.

When the researchers analysed impulsivity among the subjects, most of the differences between cannabis users and healthy controls went away, suggesting that it may play a role in performance differences.