How Cannabis Adverse Events Changed After Canada Legalized Recreational Use | Research

The takeaway

About one-third of Canadian cannabis users reported adverse events before and after legalization, with little change in overall rates but a shift toward more ER visits post-legalization.

Policymakers evaluating legalization outcomes, public health officials, and emergency medicine providers.

~33% reported adverse events; ER visits increased post-legalization while edible problems decreased

What the researchers found

Approximately one-third of cannabis users reported at least one adverse event in the past year, and 5% sought medical help. The prevalence was similar before (2018) and after legalization (2019-2021), but ER visits increased post-legalization while edible-related help-seeking decreased.

Why it matters

This is one of the first studies tracking adverse events across the legalization transition in Canada, providing real-world data on whether legalization changes the pattern of harms.

The numbers in context

18,285 respondents across 4 waves. ~33% reported at least one adverse event. 5% sought medical help. ER proportion increased (F=2.77, p=0.041). Edible-related help-seeking decreased (p=0.001).

How the study worked

Cross-sectional analysis of four survey waves from the International Cannabis Policy Study (2018-2021) covering 18,285 Canadian respondents aged 16-65 who reported cannabis use.

What this study cannot tell us

Self-report data subject to recall bias. Online survey may not represent all users. Cannot determine whether changes are caused by legalization or other factors.

How to read the evidence

Large sample across the legalization transition, though online self-report and inability to control for confounders limit certainty.

When this study was published

Published in 2024 with data from 2018-2021.

The bigger picture

The stable rate of adverse events suggests legalization did not increase harms, but the shift to ER visits may reflect changing product types or greater willingness to seek formal help.

Questions still open

Does the decrease in edible-related adverse events reflect better dosing information in the legal market?
Will adverse event patterns stabilize as the legal market matures?

Common questions

Did legalization make cannabis use more dangerous?

Adverse event rates were similar before and after legalization. ER visits increased but edible-related problems decreased, suggesting shifting patterns rather than increased overall harm.

What are the most common cannabis adverse events?

The most common reasons for seeking medical help were panic attacks, dizziness/fainting, heart/blood pressure problems, and nausea/vomiting.

Read the original research

Adverse outcomes of cannabis use in Canada, before and after legalisation of non-medical cannabis: cross-sectional analysis of the International Cannabis Policy Study.

BMJ open, 14(1), e077908

View on PubMed ↗Publisher’s paper ↗

Citation

Marquette, Anastasia; Iraniparast, Maryam; Hammond, David. (2024). Adverse outcomes of cannabis use in Canada, before and after legalisation of non-medical cannabis: cross-sectional analysis of the International Cannabis Policy Study.. BMJ open, 14(1), e077908. https://doi.org/10.1136/bmjopen-2023-077908