McGill research finds significant driving impairment 5 hours after cannabis use

MONTREAL — Driving under the influence of cannabis remains dangerous even five hours after use, a new study by McGill University researchers has found.

The study published Monday in the Canadian Medical Association Journal found that subjects who consumed cannabis had difficulty performing certain manoeuvres and were at greater risk of crashing a vehicle.

Researchers recruited people aged 18 to 24 who were already recreational cannabis users. They were tested on a driving simulator at four stages: before inhaling a regular dose of cannabis, then one, three and five hours afterwards.

Under the effect of cannabis, participants were able to perform simple driving tasks such as braking and steering. They even showed increased vigilance one hour after consumption.

But when confronted with slightly more complicated scenarios — parking between two cars at a shopping centre, passing through an intersection or avoiding pedestrians and cyclists — they had trouble.

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“There were a lot of drivers … who took part in the test and did not manage to do those manoeuvres,” said Marco Harrison, director of the CAA-Quebec Foundation.

The study, conducted with the Research Institute of the McGill University Health Centre, was funded by the Canadian Automobile Association.

A majority of the 45 people in the study said they did not feel they were in a condition to drive within five hours of consumption. Harrison said the amount of cannabis consumed — 100 milligrams — was less than would be found in a typical joint.

Isabelle Gelinas, a professor in McGill’s school of physical and occupational therapy who was co-author of the study, said there was no significant improvement in drivers’ abilities five hours after they consumed.

“At all levels, we did not necessarily see a marked difference, even after five hours,” she said. “Their capacities were quite impaired, which tells us they could not take the wheel.”

The study did not look at effects beyond five hours, meaning a person could remain unfit to drive for even longer. With legalization coming to Canada on Wednesday, the authors said their findings support a recommendation that people wait at least six hours after use before driving.

Caroline St-Pierre, The Canadian Press