Cannabis use doubles risk of a motor vehicle crash

Drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a motor vehicle crash (MVC) as those who are not under the influence of drugs or alcohol.

Key Points:

Dr. Mark Asbridge, associate professor in Dalhousie Medical School's departments of Community Health & Epidemiology and Emergency Medicine, completed a systematic review and meta-analysis of nine studies to determine if cannabis use increases the risk of a MVC. Findings were published today on the British Medical Journal's website, bmj.com.
The review, using a total sample of 49,411 people, showed that if cannabis is consumed before driving a motor vehicle, the risk of collision is nearly doubled.
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 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 cannabis.
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 tested positive for cannabis.

Pull Quotes:

"This is the first review looking at observational studies concerned with the risk of vehicle collision after the recent consumption of cannabis. This research clearly demonstrates that recent cannabis consumption impairs the skills necessary for safe driving and increases collision risk." -- Dr. Mark Asbridge, Dalhousie Department of Community Health & Epidemiology and the Dalhousie/Capital Health Department of Emergency Medicine
"This research supports existing policies, in Canada and abroad, that prohibit driving under the influence of cannabis. 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." -- Dr. Mark Asbridge, Dalhousie Department of Community Health & Epidemiology and the Dalhousie/Capital Health Department of Emergency Medicine