Canadian legalization slowed the natural decline in cannabis use among young adults compared to US peers
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- Legalization attenuated the natural "aging out" of cannabis use in Canadian young adults
- Evidence Grade:
- Quasi-experimental design with doubly robust DiD estimation and propensity balancing, though limited to two cities and short follow-up.
- Study Age:
- Published in 2025, covering March 2018 to March 2020.
- Original Title:
- Evaluating the impact of Canadian cannabis legalization on cannabis use outcomes in emerging adults: Comparisons to a US control sample via a natural experiment.
- Published In:
- The International journal on drug policy, 136, 104686 (2025)
- Authors:
- Doggett, Amanda(7), Belisario, Kyla L(2), McDonald, André J(8), Gohari, Mahmood, Leatherdale, Scott T, Murphy, James G, MacKillop, James
- Database ID:
- RTHC-06360
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Did legalization increase cannabis use?
Not directly. Both Canadian and American young adults were using less cannabis over time. But the Canadians showed smaller decreases, meaning legalization appeared to slow the natural developmental decline in use rather than causing new use.
Why compare Hamilton and Memphis?
Researchers needed a US city where no cannabis policy changes occurred during the study period to serve as a control group for the natural experiment of Canadian legalization.
What is "aging out"?
Young adults typically reduce substance use as they transition into adult roles (career, relationships, parenting). This natural decline is considered a protective factor, and the study found legalization may slow this process for cannabis.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-06360APA
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
MLA
Doggett, Amanda, et al. "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, 2025. https://doi.org/10.1016/j.drugpo.2024.104686
RethinkTHC
RethinkTHC Research Database. "Evaluating the impact of Canadian cannabis legalization on c..." RTHC-06360. Retrieved from https://rethinkthc.com/research/doggett-2025-evaluating-the-impact-of
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.