A Vancouver emergency-room doctor is warning drivers to avoid marijuana after his research has shown just over 7 per cent of those injured behind the wheel consume cannabis in the hours leading up to a crash.

Jeff Brubacher, who works at Vancouver General Hospital, published an opinion piece in this month's British Columbia Medical Journal detailing how four emergency rooms across the province surveyed 1,097 drivers and found cannabis was the most common recreational drug, after alcohol, used among injured drivers, with 7.3 per cent consuming it in the hours preceding their crashes and 12.6 per cent still showing traces of the drug from earlier use.

That same data showed 17.8 per cent of the injured drivers drank and 15.8 per cent had a blood-alcohol concentration greater than 0.08.

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Dr. Brubacher and his research team are still analyzing another 1,000 blood samples, but he said it is important the public knows past studies have suggested heavy cannabis use nearly doubles a driver's risk of having a crash resulting in serious injury or death.

"Cannabis slows reaction times, causes weaving, creates difficulty maintaining a constant speed and predisposes to distraction," Dr. Brubacher stated in his article.

He said the latest available data comes from 2012, but Vancouver's police department says it is concerned an increase in the number of people who consume cannabis recreationally or medically, as evidenced by the explosion of dispensaries over the past two years, inevitably will lead to more drivers being impaired by the drug.

Dr. Brubacher said his research has shown that cannabis was most commonly used by men and drivers under 30. The message to avoid drinking and driving has seemed to stick with younger generations of B.C. drivers, he said.

"But there are still quite a few who think 'well cannabis isn't such a big problem, drugs aren't such a big problem,' and they may be using drugs and driving instead of drinking and driving," he said.

"Just separate driving from using pot, so wait at least four hours after you use before you drive and don't mix it with alcohol."

Vancouver police spokesperson Sergeant Randy Fincham said the department hasn't compiled any data to see whether officers are catching more people driving while impaired by marijuana, but about one drug-impaired driver was caught for every three drunk drivers in last December's annual counterattack blitz.

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"With drivers that were coming through random road checks at locations in Vancouver, we were seeing drivers that were impaired by drugs, which the vast majority of the time turned out to be marijuana," Sgt. Fincham said.

There is no roadside test, like a breathalyzer, that police can use to determine whether a driver is impaired by cannabis, but each roadblock usually has an officer trained in drug recognition that analyzes a driver's speech, balance, driving behaviour, response time and odour, Sgt. Fincham said.

The "vast majority of the time" a driver impaired by drugs and caught by police will be issued a 24-hour driving prohibition, but in more serious cases involving injuries or death, officers pursuing a criminal-charge approval can demand a blood sample, Sgt. Fincham said.

Lawyer Paul Doroshenko, who specializes in impaired-driving cases, said Dr. Brubacher's ongoing study can't prove causation between pot use and crashes and has seen studies that show "there are people who are chronic users or regular users of marijuana who have no detectable impairment in their ability to drive."

Mr. Doroshenko said he hasn't seen an uptick in clients accused of driving while under the influence of pot because it's very difficult to prosecute people impaired by a drug other than alcohol.

Still, police can issue a 24-hour driving prohibition for drugs, which has no dispute process laid out by the Motor Vehicle Act, he said.

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"It's very harsh if you end up with that on your driving record. You can forget any job that ever requires driving, because nobody will ever hire you," Mr. Doroshenko said. "The cop thinks he's just doing you a favour, getting you off the road so you don't have an accident, and you end up with that."