The dangers of taking the "high" road

Increased collision risk, skill impairment

- February 14, 2012

Dr. Mark Asbridge, from the Department of Community Health & Epidemiology. (Dal Medical School photo)
Dr. Mark Asbridge, from the Department of Community Health & Epidemiology. (Dal Medical School photo)

You may have come across someone claiming they can drive just fine after marijuana use. But new Dal research suggests that if you’re the sober passenger in that scenario, you should really ask for the keys.

Dr. Mark Asbridge, associate professor in Dalhousie Medical School's Department of Community Health & Epidemiology, completed a systematic review and meta-analysis of nine studies to determine if cannabis use increases the risk of a motor vehicle collision (MVC).

His review, consisting of a total sample of 49,411 people, found that drivers who consume cannabis within three hours of driving are nearly twice as likely to cause an MVC as those who aren't under the influence of drugs or alcohol.

Dr. Asbridge's findings were published last week in the British Medical Journal.

"This is the first review looking at observational studies concerned with the risk of vehicle collision after the recent consumption of cannabis," says Dr. Asbridge, who also has an appointment in the Dalhousie/Capital Health Department of Emergency Medicine.

"This research clearly demonstrates that recent cannabis consumption impairs the skills necessary for safe driving and increases collision risk."

All MVCs involved in the study took place on a public road and involved one or more moving vehicles such as cars, vans, sports utility vehicles, trucks, buses and motorcycles. Cannabis use was identified through blood samples or direct self-report.

Rates of cannabis use high among young drivers

Rates of driving under the influence of cannabis have increased in Canada in recent years, with about four per cent of adult drivers reporting use prior to getting behind the wheel. The rate among youth and young adult drivers is higher, with between 14 and 21 per cent reporting that they have driven after using the substance.

"This research supports existing policies, in Canada and abroad, that prohibit driving under the influence of cannabis," says Dr. Asbridge. "While public health efforts aimed at drinking and driving have been highly successful, greater attention needs to be directed to cannabis and other substances that may affect safe driving performance."

Cannabis is the most widely used illicit substance globally, and recent statistics have shown a significant increase in use across the world.

Between 2000 and 2007, cannabis was the second most commonly found drug among fatally injured drivers in Canada. A 2010 roadside survey in British Columbia showed that 5.8 per cent of the 2,442 drivers stopped had cannabis in their systems.


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