Study of the Day: Smoking Pot Doubles Chances of a Car Crash

A new meta-analysis of nine studies suggests that the consumption of cannabis impairs motor tasks that are important to safe driving.

PROBLEM: Previous observational studies have failed to separate the effects of alcohol and marijuana use on the risk of vehicle collision.

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METHODOLOGY: Researchers from Dalhousie University in Canada reviewed nine previous studies with a total sample of 49,411 people to see if the consumption of cannabis, the world's most widely used illicit substance, increases the risk of a motor vehicle collision. They focused on trials that involved crashes that took place on a public road and involved one or more moving vehicles, including cars, trucks, and motorcycles. They also concentrated on studies that included driver blood samples and direct self-reports.

RESULTS: The vehicle collision risk of people who smoke pot within three hours of driving is nearly double the risk of drivers who are unencumbered by drugs or alcohol. There is also a substantially higher chance of an accident for motorists aged 35 or younger.

CONCLUSION: Marijuana users are twice as likely to cause a car crash as sober drivers.

IMPLICATION: The consumption of cannabis impairs motor tasks that are important to safe driving.

SOURCE: The full study, "Acute Cannabis Consumption and Motor Vehicle Collision Risk: A Systematic Review of Observational Studies," is published in the British Medical Journal.

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