Marijuana Impairs Driving Long After High Has Worn Off: Study

Amid increasing reports of marijuana-related accidents, as use has been legalized or decriminalized throughout the United States in recent years, new research suggests that the side effects of marijuana impair driving, even when an individual is no longer “high” or feeling the effects.

Researchers from McLean Hospital in Belmont, Massachusetts indicate that the effects of regularly using marijuana, especially if use begins earlier than the age of 16, carry over to a person’s life even when they are sober. Regular marijuana use can especially affect a person’s driving skills they found, making them unsafe drivers even when they are not high.

The findings were published this week in the journal Drug and Alcohol Dependence.

A lot is known about the dangers of driving while intoxicated, whether from alcohol or drugs, including recreational or medicinal marijuana. However, researchers wanted to know if the effects carried over when the users were no longer under the influence.

Researchers used a customized driving simulator on non-intoxicated people who were heavy, recreational cannabis users. But the users were not intoxicated when they drove in the simulator based on urine tests and had not used marijuana for at least 12 hours. They compared their driving to healthy people who had no history of cannabis use.

Marijuana users demonstrated impaired driving compared to non-users, even though they were sober. They consistently performed worse on the driving simulations than non-users.

Heavy marijuana users tended to hit more pedestrians, went over the speed limit more often, made fewer stops at red lights, and crossed the center line more often during the simulation compared to people who didn’t use marijuana at all.

Overall, heavy marijuana users failed to follow driving rules more often, even though they were sober at the time.

Heavy use was considered daily or near daily marijuana use, a minimum of four or five times per week with a lifetime exposure of 1,500 times. Researchers categorized marijuana users into groups based on the age they began regular marijuana use.

People who began smoking marijuana when they were younger than 16 showed significant driving impairment compared to those who began using after they were 16 years old.

The brain continues to develop past the age of 16, well into young adulthood. Exposing the brain consistently to substances that can impair cognition and neurodevelopment can be detrimental to the brain’s full development.

Researchers said impulsivity had a significant impact on driving performance. They found that people who use marijuana tend to be more impulsive, thus becoming unsafe drivers and disobeying more traffic laws.

When they analyzed impulsive driving actions separately and removed those, the differences between users and nonusers was low and their driving skill was roughly similar.

Recreational and medical use of marijuana is on the rise. The proportion of drivers involved in fatal crashes while high on marijuana increased from 8% in 2013 to 17% in 2014.

A recent study conducted by the U.S. Centers for Disease Control and Prevention found that 5% of the population in the U.S. admits to driving while under the influence of marijuana. That equates to roughly 12 million Americans over the age of 16 being high while driving.