Drinking and driving is never a good idea, and neither is smoking pot and driving, a study finds. People who smoke marijuana within a few hours of getting behind the wheel may be almost twice as likely to cause an accident compared with those who are sober.

A review of nine studies on pot smoking and car crashes was done by researchers from Dalhousie University in Nova Scotia, Canada. The authors wrote that previous studies have been somewhat inconclusive about marijuana’s effect on automobile collisions, some showing it linked with a higher risk of crashes, and some showing a lower risk.

The research included in the meta-analysis included observational studies of drivers who had been treated for serious injuries following a crash, or who had been part of a fatal crash. Those crashes took place on public roads and included at least one moving vehicle, such as a car, van, truck, motorcycle or snowmobile. Evidence of marijuana was found via blood tests or self-reported drug use. The studies represented a sample of 49,411 people.

When results from the nine studies were grouped together, the risk of driving under the influence of marijuana was nearly twice that of driving while unimpaired. In seven of the nine studies the risk of a crash went up when drivers had smoked marijuana within a few hours of the accident. The other two studies found that the risk of having a collision while under the influence was lower than that of sober drivers.


The study was released this week in the British Medical Journal.