Comparing Canadian and American young adults over two years, Canadian legalization was associated with smaller reductions in cannabis use and consequences than expected, reflecting attenuated "aging out" rather than increased use, with effects appearing 6-12 months post-legalization.
Policymakers evaluating legalization impacts, parents of young adults, and substance use researchers.
Legalization attenuated the natural "aging out" of cannabis use in Canadian young adults
What the researchers found
Against a general trend of decreasing cannabis use in both countries, Canadian young adults showed significantly higher cannabis use frequency at 6 months (ATT 0.22) and 12 months (ATT 0.31) post-legalization compared to the US control group. Cannabis-related consequences were also greater at both time points. These differences reflected less steep declines over time (attenuated aging out) rather than absolute increases. Alcohol outcomes showed no legalization effect.
Why it matters
This is one of the most rigorous evaluations of cannabis legalization effects, using a US comparison group and DiD methodology to approximate a natural experiment. The finding that legalization attenuated "aging out" of cannabis use is important because this natural developmental decline is a key protective factor.
The numbers in context
Canadian vs US difference at 6 months: cannabis frequency ATT 0.22 (95% CI 0.02-0.43), consequences ATT 0.76 (95% CI 0.08-1.44). At 12 months: frequency ATT 0.31 (95% CI 0.05-0.57), consequences ATT 1.04 (95% CI 0.19-1.89). No significant alcohol effects at any time point.
How the study worked
Two cohorts of emerging adults from Hamilton, Ontario, and Memphis, Tennessee, were followed every 4 months from March 2018 to March 2020 (three pre-legalization, four post-legalization assessments). Doubly robust difference-in-differences estimation with propensity score balancing compared cannabis use frequency and consequences. Alcohol outcomes served as a control.
What this study cannot tell us
Only two cities compared (Hamilton and Memphis), limiting generalizability. Pre-legalization period was short (3 waves). Follow-up ended March 2020 (start of COVID), precluding longer-term assessment. Cultural and economic differences between cities may not be fully captured by propensity matching.
How to read the evidence
Quasi-experimental design with doubly robust DiD estimation and propensity balancing, though limited to two cities and short follow-up.
When this study was published
Published in 2025, covering March 2018 to March 2020.
The bigger picture
The concept of attenuated aging out is a subtle but important finding. Young adults typically reduce substance use as they take on adult responsibilities. If legalization slows this natural process even slightly, the cumulative public health impact could be significant over time as more cohorts pass through early adulthood in a legal cannabis environment.
Questions still open
- Has the attenuated aging out continued or reversed in subsequent years? Would results differ in cities with more established legal cannabis markets? Does slowed aging out translate to long-term higher cannabis use in adulthood?
Common questions
Did legalization increase cannabis use?
Why compare Hamilton and Memphis?
What is "aging out"?
Read the original research
Evaluating the impact of Canadian cannabis legalization on cannabis use outcomes in emerging adults: Comparisons to a US control sample via a natural experiment.
The International journal on drug policy, 136, 104686
Citation
Doggett, Amanda; Belisario, Kyla L; McDonald, André J; Gohari, Mahmood; Leatherdale, Scott T; Murphy, James G; MacKillop, James. (2025). Evaluating the impact of Canadian cannabis legalization on cannabis use outcomes in emerging adults: Comparisons to a US control sample via a natural experiment.. The International journal on drug policy, 136, 104686. https://doi.org/10.1016/j.drugpo.2024.104686