Cannabis Legalisation Linked to More Adult Use and Increased Acute Harms, Review Finds

A synthesis of 14 systematic reviews finds consistent rises in adult cannabis use, emergency visits, and paediatric poisonings after legalisation in the USA and Canada, while adolescent use trends remain mixed and long-term consequences remain largely unknown.

Why This Matters

Cannabis legalisation is expanding globally, with dozens of jurisdictions weighing policy changes based heavily on the North American experience. The public health consequences of these policy shifts are no longer theoretical: nearly a decade of post-legalisation data now exists across 24 US states and all of Canada. Yet the evidence remains fragmented across outcome domains and study designs, making structured synthesis essential for clinicians and policymakers. This review, published on a high-profile platform, is likely to shape regulatory debates in Europe, Australasia, and Latin America in the near term.

Clinical Summary

Published in The Lancet Public Health in February 2025, this narrative review synthesises findings from 14 systematic reviews covering health outcomes following adult non-medical cannabis legalisation in North America. The authors apply a methodological framework modelled on the US National Academies of Sciences, Engineering, and Medicine approach to grade the strength of evidence across multiple domains, including cannabis use prevalence, emergency department visits, poisonings, cannabis use disorder, maternal use, and criminal justice outcomes. The underlying biology of concern centres on the known acute toxicity of concentrated THC products, particularly edibles, which produce delayed-onset effects that increase the risk of overconsumption, and on the vulnerability of developing brains in children and adolescents to cannabinoid exposure.

The most consistent quantitative finding is a substantial increase in acute poisonings after legalisation, with a pooled relative risk of 3.56 (95% CI 2.43 to 5.20) and an even larger effect in paediatric populations (RR 4.31, 95% CI 2.30 to 8.07). Adult cannabis use rose modestly, with Canadian past-three-month prevalence increasing from 14% in 2018 to 20% in 2020, concentrated in the 25 to 44 age group. Adolescent use did not consistently increase in cross-sectional surveys, though some longitudinal studies detected post-legalisation rises among young adults. The authors acknowledge substantial limitations, including short follow-up periods, heterogeneous regulatory models across jurisdictions, and the inability to attribute population-level changes causally to legalisation given the observational study designs underlying their included reviews.

Dr. Caplan’s Take

This review does a useful job of compiling what we can actually observe in the wake of North American cannabis legalisation, and the signal around paediatric poisonings and emergency department presentations is genuinely concerning and clinically actionable. But this is a narrative review built on observational data, and the authors bring an explicit framing that commercial cannabis markets amplify harm. That framing is not unreasonable, but it does shape which evidence gets emphasized. When patients ask me whether legalisation has been shown to be harmful, the honest answer is that certain acute harms have clearly increased, while other feared outcomes, particularly adolescent use, have not materialized as consistently as predicted.

In practice, I counsel patients and families about the specific risks that this evidence does support: the danger of edible products around children, the importance of secure storage, and the reality that concentrated THC products carry dose-dependent risks for psychiatric symptoms, especially in younger adults. I do not treat this review as grounds for or against any particular policy position, but I do use its findings to anchor concrete harm-reduction conversations, particularly around product type, dosing, and household safety.

Clinical Perspective

This review sits at an intermediate point in the research arc. It confirms what earlier individual studies suggested about acute poisoning risk and emergency presentations, now supported by pooled estimates from a high-quality systematic review. It challenges the assumption that adolescent use would inevitably rise with legalisation, though the mixed findings and short follow-up periods mean this question remains genuinely unresolved. Clinicians should note that the evidence does not yet support strong claims about long-term outcomes such as cannabis use disorder prevalence, psychosis incidence, or cardiovascular risk in the post-legalisation population. What it does support is heightened vigilance around acute presentations, particularly in paediatric and young adult populations.

From a safety standpoint, the sharpest clinical signal involves edible cannabis products, which were associated with disproportionate increases in paediatric poisoning after their commercial introduction in Canada. Clinicians working with families should actively screen for cannabis product accessibility in the home, much as they would for medications or household chemicals. For young adults presenting with acute psychiatric symptoms, cannabis exposure should be considered in the differential, especially in jurisdictions with legal retail access. The single most actionable recommendation from this evidence is to incorporate cannabis product safety counselling, including secure storage and awareness of edible product risks, into routine anticipatory guidance for families with young children.

Study at a Glance

Study Type

Narrative review of systematic reviews (Health Policy paper)

Population

Adults, adolescents, and children in 24 US legalising states and Canada

Intervention

Adult non-medical cannabis legalisation and commercial retail implementation

Comparator

Pre-legalisation periods and non-legalising jurisdictions

Primary Outcomes

Cannabis use prevalence, emergency department visits, poisonings, cannabis use disorder, arrests

Sample Size

14 systematic reviews synthesised (12 moderate-quality, 2 high-quality)

Journal

The Lancet Public Health, Volume 10, February 2025, e148-159

Year

2025

DOI or PMID

Not provided in available text

Funding Source

Not specified in available text

What Kind of Evidence Is This

This is a narrative review published as a Health Policy paper, not a systematic review. It synthesises conclusions from 14 systematic reviews using a NASEM-style evidence-grading framework, but it lacks a pre-registered search protocol and formal risk-of-bias assessment. Its position in the evidence hierarchy is therefore below systematic reviews and meta-analyses: its authority depends entirely on the quality and representativeness of the reviews it chose to include. The single most important inference constraint is that narrative selection of source reviews may introduce bias in which findings are emphasized or omitted.

How This Fits With the Broader Literature

The findings on acute poisonings align closely with the high-quality systematic review by Allaf and colleagues (2023), which provided the pooled relative risk estimates cited here, and with earlier work by Myran and colleagues documenting sharp increases in paediatric cannabis poisonings following edible product availability in Ontario. The mixed evidence on adolescent use is consistent with findings from the Monitoring the Future survey and the Youth Risk Behavior Survey, both of which have generally failed to detect consistent post-legalisation increases in adolescent cannabis use at the national level, though regional variation exists.

Where this review extends prior work is in attempting to map evidence quality across multiple outcome domains simultaneously, providing a structured overview that individual systematic reviews do not offer. However, its framing around commercial alcohol-model regulation as inherently harmful places it within a specific policy perspective that not all researchers in this field share, and readers should consider how this framing shapes the narrative emphasis.

Common Misreadings

The most likely overinterpretation is to conclude that cannabis legalisation has been proven to cause the observed increases in emergency visits and poisonings. The underlying evidence is entirely observational, and the review itself acknowledges that increased detection, reduced stigma around reporting cannabis use, and secular trends in cannabis potency and product availability all confound causal attribution. Additionally, a reader might conclude that the absence of consistent adolescent use increases means legalisation is safe for young people, but the short follow-up periods and methodological heterogeneity across studies mean this question remains genuinely open rather than resolved in either direction.

Bottom Line

This narrative review provides a structured overview of the observable public health associations following cannabis legalisation in North America. It establishes that adult use has risen modestly, acute harms including paediatric poisonings and emergency presentations have increased substantially, and adolescent use effects remain unclear. It does not establish causation, does not assess long-term outcomes, and reflects a specific policy framing. For clinicians now, the actionable finding is the need for routine counselling on edible product safety and secure storage in households with children.

References

1. The Lancet Public Health. Cannabis legalisation and public health effects in the USA and Canada: a narrative review. Lancet Public Health. 2025;10:e148-159.
2. Allaf S, et al. Cannabis-related poisonings following legalisation of non-medical cannabis: a systematic review and meta-analysis. Systematic Reviews. 2023. (Cited as high-quality systematic review providing pooled RR estimates.)
3. Myran DT, et al. Unintentional pediatric cannabis exposures after legalization of recreational cannabis in Canada. JAMA Network Open. (Cited for Ontario paediatric poisoning data post-edible availability.)
4. National Academies of Sciences, Engineering, and Medicine (NASEM). The health effects of cannabis and cannabinoids: the current state of evidence and recommendations for research. Washington, DC: The National Academies Press; 2017.