Summary: Removing criminal penalties for personal cannabis possession or adopting tightly regulated legal frameworks does not, on its own, increase overall use. In contrast, large-scale, for-profit commercial markets—such as those established in parts of the United States and Canada—are linked to higher-product potency, rising addiction rates and greater mental health-related hospital admissions. A separate review of 54 clinical trials found limited evidence that medical cannabinoids are effective for primary psychiatric disorders such as anxiety, post-traumatic stress disorder (PTSD) or depression.
Key Facts
- Policy design matters: Decriminalization of possession or tightly regulated legal access (with limits on potency and market scale) shows little evidence of driving population-level increases in cannabis use.
- Commercialization increases harm: For-profit legal markets have been associated with greater product potency and higher rates of cannabis use disorder and daily consumption.
- Mental health impacts: Commercialized markets correlate with increased hospital admissions for psychosis, particularly cases where severe psychotic disorders occur alongside cannabis use disorder.
- Justice and equity: Shifting from criminal enforcement to health-focused approaches through decriminalization can reduce racially disproportionate policing that has historically affected Black communities.
- Limited medical benefit: Across 54 clinical trials, cannabinoids showed modest benefits for insomnia and some autism-related traits and reduced cannabis withdrawal in specific contexts, but provided no meaningful evidence of effectiveness for anxiety, PTSD, depression or opioid dependence.
Source: University of Bath
Key finding: Removing criminal penalties for personal cannabis possession or introducing tightly controlled legal access does not appear to increase population-level cannabis use. However, when legalization is implemented through wide-scale, for-profit markets, evidence shows greater availability, higher potency products and increased rates of dependence and related harms.
These conclusions come from a study published in The Lancet Psychiatry on 17 June, led by addiction and mental health researchers at the University of Bath with international collaborators from the Americas, Europe, Africa, Australia, New Zealand and Asia. Co-authors Professor Tom Freeman and Dr Rachel Lees Thorne emphasize that how policy changes — not merely whether they change — determines public health outcomes.
Evolving global policies
Across the world, cannabis policy now ranges from strict prohibition to fully commercialized legalization. The review assesses policy shifts between 2000 and 2025 and examines links to cannabis consumption, cannabis use disorder and other psychiatric outcomes.
In some jurisdictions, such as Uruguay, legalization has been implemented with tight controls: adults can access a limited range of lower-potency products through pharmacies, participate in regulated cannabis social clubs or grow a limited number of plants at home. Where legal supply is strictly regulated, there is little evidence of substantial increases in use.
By contrast, several US states and Canada have created large, for-profit retail markets with broad product ranges and increased marketing. In these commercialized systems, studies show higher prevalence of adult cannabis use, growth in daily use, rising potency of products and elevated rates of cannabis use disorder. In the US, recent data indicate daily cannabis consumers now outnumber daily alcohol consumers in some surveys.
Professor Freeman noted: “The type of policy change is critical. Decriminalization or tightly regulated legalization can remove the harms associated with criminal penalties while limiting increases in use. But commercialized legalization can prioritize corporate interests, expand availability, increase product strength and encourage marketing practices that raise public-health risks.”
Medical cannabis and clinical evidence
The review also evaluated medical cannabis access and the clinical evidence for cannabinoids in psychiatric care. Researchers caution that poorly regulated medical supply, especially when introduced without solid safety and efficacy data, may expose patients to unnecessary risks.
A systematic synthesis of 54 randomized trials found limited benefits from medical cannabinoids in psychiatric contexts. Positive, though modest, effects were observed for improving sleep in insomnia, reducing cannabis withdrawal symptoms in some settings, and alleviating certain tics and traits associated with autism. However, cannabinoids showed no consistent or clinically meaningful benefits for anxiety disorders, PTSD, psychosis or opioid dependence, and no randomized trials addressed depression as a treatment target.
The authors highlight the importance of careful regulation and ongoing monitoring of medical cannabis programs to avoid unintended harms and to ensure therapies are supported by clear evidence of safety and effectiveness.
Cannabis and psychosis
The evidence suggests daily cannabis use can interact with other risk factors to elevate the likelihood of psychosis. While the review did not find consistent, direct links between policy change and the overall incidence of psychotic disorders, commercialized markets were associated with increased hospital admissions for psychosis and more cases where psychotic disorders co-occurred with cannabis use disorder.
Key Questions Answered:
A: No. Analysis of international data from 2000–2025 shows that decriminalizing possession for personal use did not cause a rise in population-level consumption. Decriminalization tends to shift responses from criminal enforcement toward healthcare without expanding the user base.
A: The evidence points to commercialization and for-profit market structures. Increased availability, aggressive marketing and higher-THC products in commercial markets contribute to greater use, greater dependence and higher rates of psychosis-related hospital admissions.
A: A review of 54 clinical trials found little strong evidence that cannabinoids are effective for anxiety, PTSD or opioid dependence, and no completed trials addressed depression. Modest benefits were reported for insomnia, reducing cannabis withdrawal and specific autism traits, but overall clinical support for psychiatric indications is limited.
Editorial Notes:
- Article edited by a Neuroscience News editor.
- Journal paper reviewed in full by the editorial team.
- Additional contextual information added by staff.
About this psychosis and cannabis research news
Author: Vittoria D’Alessio
Source: University of Bath
Contact: Vittoria D’Alessio – University of Bath
Image: Image credited to Neuroscience News
Original research: Freeman TP, Lees Thorne R, Wadsworth E, Carney T, Castillo-Carniglia A, Cerdá M, Kalayasiri Q, Kilmer B, Lorenzetti V, Manthey J, Myran DT, Rivera-Aguirre A, Rychert M, Wilson J, Yimer T, Hall W. International cannabis policies and their association with cannabis use, cannabis use disorder, and other psychiatric disorders. Lancet Psychiatry. DOI: 10.1016/S2215-0366(26)00087-8. Open access.
Abstract
International cannabis policies and their association with cannabis use, cannabis use disorder, and other psychiatric disorders
Cannabis policies, ranging from prohibition to commercialized legalization, have changed rapidly across countries. This review examines associations between international policy shifts from 2000–2025 and outcomes including cannabis use prevalence, cannabis use disorder and psychiatric disorders. Commercialized non-medical markets in Canada and the USA were linked to increased adult cannabis use, higher rates of cannabis use disorder and greater product potency. There was no consistent evidence tying policy change directly to overall incidence of psychotic disorders, but commercialized legalization was associated with greater hospital admissions for psychosis and for psychotic disorders comorbid with cannabis use disorder. Poorly regulated medical access without solid efficacy and safety data may pose additional risks. Policies that limit commercialization, such as tightly regulated medical or non-medical legalization, showed weaker associations with increases in use or psychiatric disorders, though longer-term evaluation is required. Limited evidence suggested decriminalization in regions of Europe, Africa, Oceania and Asia was not associated with population-level increases in cannabis use or psychiatric disorders.