Over Half of Injured Canadian Drivers Had Drugs or Alcohol in Their Blood — THC Was the Most Common Drug | Research

The takeaway

Blood tests from 8,328 injured drivers at 15 Canadian trauma centers found 54.9% had at least one substance detected, with THC being the most commonly found drug after alcohol.

Read this if you follow cannabis policy and traffic safety — this is one of the largest post-legalization datasets on substance-involved driving injuries.

What the researchers found

This is one of the largest and most rigorous studies of substance use among injured drivers ever conducted. Rather than relying on self-report or police records, researchers prospectively collected blood samples from 8,328 injured drivers treated at 15 Canadian trauma centers from 2019 to 2023 — spanning the period after Canada's 2018 recreational cannabis legalization.

The headline: 54.9% of injured drivers had at least one impairing substance in their blood. More than half. The specific breakdown reveals the scale of the problem across substance classes.

Alcohol was the most prevalent, as expected. But THC was the most commonly detected drug (after alcohol), confirming that cannabis-involved driving is a significant component of the impaired driving landscape in post-legalization Canada. Stimulants, opioids, and depressants were also detected, often in combination with other substances.

The study identified demographic and collision factors associated with substance use, and notably compared prevalence across different parts of Canada, revealing geographic variation in the drug-involved driving picture.

Polysubstance use (multiple substances simultaneously) was common, adding to the clinical complexity — the effects of THC combined with alcohol or other drugs on driving may be different from the effects of any single substance.

Why it matters

Canada legalized recreational cannabis in 2018 with the explicit goal of reducing harms, including impaired driving. This study provides the most comprehensive post-legalization snapshot of substance-involved driving injuries, showing that THC is a major part of the picture. The 54.9% overall prevalence rate is a sobering number for traffic safety policy.

The numbers in context

8,328 injured drivers. Mean age 43 years. 67.3% male. 54.9% had at least one substance detected. THC was the most common drug after alcohol. 15 trauma centers across Canada. Study period: January 2019 – June 2023 (post-legalization).

How the study worked

Cross-sectional study with prospective blood collection from 8,328 injured drivers at 15 Canadian trauma centers (January 2019 – June 2023). Blood tested for THC, alcohol, stimulants, opioids, and depressants. Demographic and collision details extracted from medical records. Crude prevalence computed overall and by subgroup. Logistic regression identified factors associated with substance use.

Who was studied

N=8,328 injured drivers aged 19-75 years, 67.3% male, Canada.

What this study cannot tell us

Only includes injured drivers who reached trauma centers — not all impaired driving incidents, and not uninjured impaired drivers. Blood collection may have been delayed (as documented in RTHC-00092 and RTHC-00124), potentially missing some THC-positive cases. Detection of a substance in blood doesn't prove impairment at the time of the crash. No pre-legalization comparison from the same trauma centers, making it impossible to determine whether legalization changed prevalence. Cannot distinguish between driver-at-fault and not-at-fault for drug involvement.

How to read the evidence

Large cross-sectional study with prospective blood collection from multiple trauma centers. The objective blood testing (rather than self-report) is a major strength. Limited by the lack of pre-legalization comparison and inability to prove impairment from detection alone.

When this study was published

Published in 2025 with data through June 2023. This represents the most comprehensive post-legalization impaired driving data from Canada.

The bigger picture

This large-scale Canadian data set complements the driving cluster in the database: RTHC-00092 (Italian DUI blood draw delays), RTHC-00093 and RTHC-00121 (self-assessed driving impairment), RTHC-00124 (Wisconsin DUI delays). While those studies examined the detection and measurement side, this study documents the sheer prevalence — over half of injured drivers testing positive for something, with THC as the top drug. The post-legalization timing makes this particularly relevant for countries and states considering cannabis legalization.

Replication

Not stated in abstract.

Funding

Not reported in abstract.

Conflicts of interest

Not reported in abstract.

Questions still open

Did cannabis legalization increase the prevalence of THC-positive injured drivers compared to the pre-legalization period? What proportion of THC-positive drivers were actually impaired versus having residual THC from prior use? Does polysubstance use (THC + alcohol, for example) produce worse crash outcomes than single-substance impairment? Are there effective public health interventions that reduce cannabis-impaired driving after legalization?

Read the original research

Prevalence of Impairing Substance Use in Injured Drivers.

JAMA network open, 8(4), e256379

JAMA Network Open is a reputable, peer-reviewed medical journal known for publishing high-quality research.

View on PubMed ↗Publisher’s paper ↗

Citation

Brubacher, Jeffrey R; Erdelyi, Shannon; Chan, Herbert; Simmons, Sarah; Atkinson, Paul; Besserer, Floyd; Clarke, David B; Davis, Phil; Daoust, Raoul; Émond, Marcel; Eppler, Jeff; Lee, Jacques S; MacPherson, Andrew; Magee, Kirk; Mercier, Eric; Ohle, Robert; Parsons, Mike; Rao, Jagadish; Rowe, Brian H; Taylor, John; Vaillancourt, Christian; Wishart, Ian. (2025). Prevalence of Impairing Substance Use in Injured Drivers.. JAMA network open, 8(4), e256379. https://doi.org/10.1001/jamanetworkopen.2025.6379