Among 29,153 medical cannabis patients in Ontario, authorization was associated with a small but significant increase in motor vehicle crash-related healthcare visits over 6 months, primarily driven by emergency department visits.
Medical cannabis program administrators, driving safety researchers, and clinicians authorizing medical cannabis.
2.92 extra MVC-related visits per 10,000 patients over 6 months
What the researchers found
After accounting for an initial decrease in MVC-related visits (-2.42/10,000 patients immediately post-authorization), there was a significant increasing trend (+0.89 events/10,000 relative to controls, p=0.0019). Overall, authorization was associated with an increase of 2.92 events/10,000 (95% CI: 0.64-5.19) over 6 months. The effect was largely driven by MVC-related emergency department visits (+0.80/10,000, p<0.001).
Why it matters
This is one of the largest studies directly linking medical cannabis authorization to traffic safety outcomes using administrative health data. The small but significant increase in MVC-related visits has public health implications as medical cannabis programs expand.
The numbers in context
29,153 patients. Immediate effect: -2.42 visits/10,000 (p=0.014). Trend effect: +0.89/10,000 per period (p=0.0019). Net 6-month increase: +2.92/10,000 (95% CI: 0.64-5.19). Driven by ED visits: +0.80/10,000 (p<0.001).
How the study worked
Matched cohort study of 29,153 medical cannabis patients and matched controls in Ontario, Canada (2014-2017). Linked to administrative health databases. Interrupted time series analysis comparing MVC-related healthcare visits 6 months before and after authorization.
What this study cannot tell us
Observational design cannot prove cannabis caused crashes. Medical cannabis patients may differ from controls in unmeasured ways. Cannot determine impairment at time of crash. Short 6-month follow-up.
How to read the evidence
Large population-based cohort with interrupted time series analysis. Strong methodology but observational limitations remain.
When this study was published
2021 study using Ontario health data from 2014-2017.
The bigger picture
The magnitude is small (about 3 extra MVC-related visits per 10,000 patients over 6 months), but at the population level of millions of cannabis users, this could translate to meaningful numbers. The initial decrease followed by gradual increase may reflect initial caution followed by normalization of driving behavior.
Questions still open
- Does the increasing trend continue beyond 6 months? Are specific patient subgroups at higher risk? Would driver education at authorization reduce MVC rates?
Common questions
Does medical cannabis increase car crash risk?
Why was there an initial decrease in crashes?
Read the original research
Cohort study of medical cannabis authorization and motor vehicle crash-related healthcare visits in 2014-2017 in Ontario, Canada.
Injury epidemiology, 8(1), 33
Citation
Lee, Cerina; Voaklander, Don; Minhas-Sandhu, Jasjeet K; Hanlon, John G; Hyshka, Elaine; Dyck, Jason R B; Eurich, Dean T. (2021). Cohort study of medical cannabis authorization and motor vehicle crash-related healthcare visits in 2014-2017 in Ontario, Canada.. Injury epidemiology, 8(1), 33. https://doi.org/10.1186/s40621-021-00321-1