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Cannabis edges alcohol in driver impairment cases, study finds

Researchers looked at more than 10,000 motor vehicle collisions in Canada
Cannabis has edged out alcohol as the most commonly detected single substance with Atlantic provinces leading the country in number of injured drivers who are more likely to have used weed, says a new study. A man holds a joint while smoking marijuana, in Vancouver on Wednesday Oct. 17, 2018. THE CANADIAN PRESS/Darryl Dyck

A six-year analysis of more than 10,000 Canadian drivers involved in motor vehicle collisions suggests cannabis has edged out alcohol as the most common impairing substance detected through after-crash blood testing.

The National Drug Driving Study 2024, produced by the University of British Columbia, says researchers analyzed blood samples from drivers in British Columbia, Alberta, Saskatchewan, Ontario, Quebec, Nova Scotia, New Brunswick and Newfoundland and Labrador between 2018 and 2023.

They found 54 per cent of these injured drivers tested positive for at least one impairing substance, and among that group 16.6 per cent had cannabis in their blood stream while 16 per cent had alcohol.

“Driving after cannabis use appears to be an emerging problem in Canada and may now be more common than driving after drinking alcohol,” the study says.

“However, given the very high crash risk associated with alcohol, and the fact that most ‘cannabis positive’ drivers had low THC (the active substance in cannabis) levels, it can be concluded that driving after drinking remains a bigger problem in Canada.”

The study also found that Atlantic Canada led the country in the proportion of injured drivers more likely to have used weed.

Of the 624 injured drivers from Nova Scotia, New Brunswick and Newfoundland and Labrador who were tested during the study period, 26 per cent of drivers tested positive for cannabis while 22 per cent tested positive for alcohol. Overall, 70 per cent tested positive for drugs or alcohol, which was also higher than the national average.

Lead author Dr. Jeff Brubacher, who is with the University of British Columbia’s department of emergency medicine, said the overall prevalence of impaired driving in Atlantic Canada was something that stood out for him.

“That was the single most striking thing,” he said. “I would say it’s a problem across the country, but it does seem to be worse in Atlantic Canada.”

Researchers in British Columbia have been studying cannabis in blood alcohol since 2012, and it’s clear that the number of drivers with weed in their system has gone up since legalization in 2018, he said.

“I would still say that good, old-fashioned alcohol is still probably the biggest problem of a single substance,” Brubacher said. “But a new problem is the combination of alcohol and cannabis, and that’s a bad combination.”

The study said cannabis intoxication causes attention deficits, slows reaction time and impairs tasks such as tracking ability — such as staying within a lane — or monitoring the speedometer. However, it says habitual cannabis users may develop tolerance to some of its effects.

“The effect of alcohol on driving and road safety is well-studied and understood,” the study says. “Unlike alcohol, it is often not possible to predict how driving will be affected at different drug-blood-alcohol concentrations.”

The target audience for this study, Brubacher said, includes emergency room physicians, public health officials, police and organizations that help spread awareness about safe driving.

“It’s just to warn people of the risks of driving while impaired, of the risks of combining alcohol and cannabis,” he said. “I’m really hoping we can continue to collect this kind of data, and I’m hoping that … police can use it to guide enforcement, and injury prevention people can use it for public education.”

READ ALSO: Early data suggests no post-legalization spike in drug-impaired driving charges