Medical Marijuana May Impair Driving Skills, Study Suggests

Medical marijuana may worsen driving skills, but typical sobriety tests do not detect the impairment, a small study suggests.

The study included 12 frequent marijuana users and 12 people who used marijuana three times a month at most. At three points, researchers gave participants a 20 milligram or 10 milligram pill of dronabinol, sold as Marinol (which contains THC, the active ingredient in marijuana), or a placebo pill that contained no drugs. After two to four hours, participants completed a series of driving tests. They tried to maintain a constant speed and drive in a straight line, or follow at a constant distance behind another vehicle. They also underwent a typical sobriety test, which includes walking and turning, and balancing on one leg.

The study found participants given medical marijuana tended to weave a few centimeters side to side in the hours after they took the drug. The impairment was smaller in those who used marijuana regularly, Reuters reports. After using medical marijuana, participants took a second or so longer to react when they followed another car. The researchers acknowledge this finding could have been due to chance.

In people who had not built up a tolerance to marijuana, the effects were similar to driving with a blood alcohol content of 0.08, the point at which drivers are considered legally impaired.

The sobriety tests did not detect any impairment due to marijuana, the researchers report in the journal Addiction.

Dr. Guohua Li of Columbia University, who has studied marijuana and traffic accidents and was not involved in the new study, told Reuters, “At this time, we know very little about the possible effects that medical marijuana may have on, say, motor vehicle crash rates, injury rates and mortality rates. There is a concern medical marijuana may interact with other drugs such as alcohol that may further compromise driving safety.”