When Mark Asbridge came across surveys indicating driving under the influence of drugs such as cannabis has surpassed the rate of driving under the influence of alcohol, it stirred his interest in exploring the precise effect of cannabis on drivers.

“People are quite concerned about this issue,” says Asbridge, an assistant professor in the Department of Epidemiology and Community Health at Dalhousie University.

Cannabis affects drivers differently than alcohol. It distorts the user’s perception of space, rather than slowing down reaction time as is the case with alcohol, Asbridge said.


“It’s the question of risk, and whether we can demonstrate if cannabis can actually increase the risk of a collision, whether or not it can impact the severity.” Asbridge says John Macdonald, co-ordinator of the Drug Recognition Program for Nova Scotia, says use of marijuana has grown in recent years and voiced concern about the potential ramifications on the road.

“You can do the typical driving functions — switching gears, steering, breaking, accelerating — but you’re unaware of your environment. When your divided attention ability is diminished by drugs, you’re not quite able to pay attention to the traffic lights, the crosswalks, or whatever’s around you,” he said. “It takes away your ability to pay attention. All drugs, whether it be cannabis or alcohol, are equal in that way.”

Over the next two years, Asbridge and the rest of his team plan to survey 1,500 people in three different Canadian hospitals, including the QEII Health Sciences Centre in Halifax, and two more hospitals located in Toronto.

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