Road accidents involving drunk drivers are different from those involving drivers who are high on cannabis, according to a Canada-wide study involving researchers from two Quebec universities.
Accidents involving alcohol are more likely to occur at night, on weekends and in rural areas. They're also more likely to involve a single vehicle and cause serious injuries.
Meanwhile, accidents involving tetrahydrocannabinol (THC), the psychoactive substance in cannabis, are more likey to occur during the day, on weekdays, and to involve several vehicles.
The data suggests alcohol and THC are not consumed in the same contexts, said one of the study's authors, Dr. Éric Mercier of the CHU de Québec-Université Laval research centre.
"This is one of the main conclusions of our study," he said. "We see a lot of probably recreational use of alcohol in slightly more festive contexts in the evening, on weekends, whereas for THC it doesn't seem to be the case."
Cannabis use, he added, appears to be often used on a routine, daily basis rather than recreationally during special events.
Fifteen trauma centres participating in the study took blood samples from drivers within six hours of an accident. Some 7,000 people were studied, and alcohol or THC was detected in the blood of a third of them.
Analyses found THC in 18 per cent of patients and alcohol in 17 per cent. Four per cent of patients had high THC levels (equal to or greater than five ng/ml), and 13 per cent had high alcohol levels (equal to or greater than 0.08 per cent).
"Police, paramedics and doctors have long known that a late-night traffic accident is likely to involve alcohol," said Mercier.
"But these factors about driving under the influence of alcohol are not at all the same for THC," he added. "And for us, on a day-to-day basis, that represents a big difference."
Five percent of drivers aged 19 and under had high levels of THC, compared with 3 per cent who had high levels of alcohol.
Men were over-represented in both categories compared to women.
"Our study shows once again that men are more at risk of having an accident in which alcohol is involved," he commented. "And then, unfortunately, it's the first time we've shown that this also seems to be the case with THC, more particularly when the level of intoxication was high."
This data should be used to guide prevention campaigns and interventions in emergency departments and elsewhere, Mercier said.
He pointed out that information circulating about the impact of THC intoxication on driving ability is "very contradictory."
"Unfortunately, yes, there is a certain trivialization (of THC) that exists," he said. "The data (...) have been very much presented as simply that there is no impact, when this is not the case. I think it's very important to say that you're a very poor judge of your level of intoxication, and then of your actual ability to drive."
He said that data doesn't necessarily imply a causal link between THC consumption and the accident.
However, since THC impacts driving ability, consuming it before getting behind the wheel "probably wouldn't give us the best chance as a driver to avoid the accident," Mercier explained.
Professors Marcel Émond of the Université Laval and Raoul Daoust of the Université de Montréal are also among the study's signatories.
The findings are published in the scientific journal Addiction.
This report by The Canadian Press was first published in French on June 28, 2023.
