Marijuana impairs driving more, and for longer, than users think, study finds

Jan. 26 (UPI) -- Smoking marijuana impacts adults' ability to drive safely, yet most who use the drug believe otherwise, a study published Wednesday by JAMA Psychiatry found.

Adults in the study performed "significantly" less well on tests designed to assess their driving abilities, including reaction times 30 minutes and 90 minutes after smoking marijuana, the data showed.

They also had "borderline" declines in driving performance 3 1/2 hours after using the drug. By 4 1/2 hours after use, driving performance returned to normal levels, the researchers said.

However, nearly 70% of marijuana smokers expressed willingness to drive within 90 minutes of smoking marijuana, and had confidence in their ability to do so safely, according to the study.

"It is important that [marijuana] users be aware that they are not always accurate regarding their self-perception of impairment," study co-author Thomas D. Marcotte told UPI in an email.

"In addition, experienced users should not assume they're less impaired simply because they have more familiarity with the product -- [they may] still be a risk on the road," said Marcotte, a professor of psychiatry at the University of California-San Diego.

Research suggests that the number of motorists "driving while high" has increased in the 18 states that have legalized marijuana across the country, particularly among teens.

Some studies have found that the drug impairs driving skills for up to four hours following use.

For this study, Marcotte and his colleagues assessed the driving skills of 191 marijuana smokers using a driving simulator test.

Smokers were tested 30 minutes, 90 minutes, three and a half hours and four and a half hours after smoking marijuana containing different levels of tetrahydrocannabinol, or THC, the ingredient that gives the drug its intoxicating effect.

Although "there is significant variability between individuals in the impairing effects of THC, our study suggests that the impairing effects were gone by 4 1/2 half hours," Marcotte said.

"This may occur sooner in some, and it may last longer in others, [but] users should be mindful that they may not always be the best judge of whether it's safe to drive," he said.