Impairing substances detected in 55% of drivers involved in an accident

By Denis Méthot, published on May 16, 2025, 2:56 p.m.

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“Impaired driving is a substantial road safety concern in Canada,” observe about twenty Canadian researchers who studied the extent of psychoactive substance use among injured drivers across the country.

According to their findings, in a sample of 8,328 injured drivers who were treated in a Canadian trauma centre after an accident, nearly 55% tested positive for a harmful substance.

These substances could be alcohol (16.1%), depressants (28.4%), tetrahydrocannabinol or THC (16.3%), stimulants (12.7%) and opioids (10.9%). Prevalence varied depending on age, sex, rurality, type of collision and region.

“Although presence of an impairing substance does not necessarily mean the driver was impaired, our findings are concerning,” the authors commented.

Like depressants, opioids and stimulants, cannabis is associated with an increased risk of road accidents.

The results of their analysis, entitled Prevalence of Impairing Substance Use in Injured Drivers, were published on April 22, 2025, in JAMA Network Open, a recognized American medical publication.

15 trauma centres in eight provinces

Blood samples from injured drivers treated in 15 trauma centres in eight provinces were analyzed. Researchers measured the blood levels of alcohol, THC—the main impairing component of cannabis—stimulants, opioids and depressants from January 2019 to June 2023.

They identified vehicle drivers (cars, sport utility vehicles, pickup trucks, vans, heavy vehicles and motorcycles) whose blood had been drawn within six hours following a road accident.

Those who died at the scene or in the emergency room, and those with minor injuries not requiring a blood sample, were excluded from the study.

Nearly 55% of drivers tested positive

The overall results show that impaired faculties still play a major role in the cause of road accidents in Canada. Of 8,328 injured drivers—5,605 men and 2,723 women—4,568 (54.9%) tested positive for an impairing substance and 1,798 (21.6%) tested positive for two or more substance classes.

Depressants, substances that slow down central nervous system activity, were the most frequently detected class of impairing substances (28.4% of drivers).

Two-thirds of the collisions (5,369) occurred on a weekday, and 3,373 (40.5%) were single-vehicle accidents.

Key findings from the Canadian study

THC was the most frequently detected substance. It was found in 16.3% of injured drivers. That is more than alcohol (16.1%). However, the presence of THC does not necessarily indicate recent cannabis use.

Researchers detected stimulants in 12.7% of injured drivers, opioids in 10.9%, and depressants in 28.4%.

Substance use trends varied with age. THC was twice as prevalent among injured drivers aged 19 to 24, likely because, the authors believe, young people think they can drive safely after using cannabis and that the risk of being caught by the police is low.

Alcohol was more prevalent among those aged 19 to 34.

Stimulants were mainly observed among those aged 35 to 44.

Opioids were mostly found among those aged 55 to 64.

Depressants were more frequently detected among those aged 65 to 74, likely reflecting higher use of prescription medications among older individuals.

Men were more likely than women to have consumed alcohol, tested positive for THC, or used more stimulants, but were less likely to have used a depressant.

The presence of depressants was more common among women.

One in eight drivers had a blood alcohol level above 0.08%.

Those under 19 were less likely to have a blood alcohol level above 0.08%. However, 7.6% exceeded this limit. “This finding contrasts with Canadian roadside surveys, which found almost no alcohol use in drivers younger than 19 years.”

Psychoactive substance use and polysubstance use were more frequent among drivers with a rural address (64.3%) than among those with an urban address (52.9%), particularly for alcohol.

Rural drivers were more likely to have consumed alcohol, stimulants, depressants, opioids, any substance or multiple substance classes than those with an urban address.

“These statistics suggest that, although more drivers test positive for THC, alcohol remains the greater threat to road safety,” the researchers commented.