The more commercial the cannabis market, the higher the health risks, study finds

A research team from the University of Bath and international colleagues has published a systematic review in The Lancet Psychiatry, pooling evidence on how cannabis policy reforms between 2000 and 2025 affected mental health outcomes.

What the data show

We found little evidence of changes in consumption after decriminalization or strictly controlled legalization.

The picture shifts in countries with fully commercialised markets. In the United States and Canada, the expansion of for-profit cannabis retail coincided with increased consumption, higher-potency products, and a growing share of adults diagnosed with cannabis addiction (defined as difficulty quitting despite negative effects on daily life). Hospital admissions for psychosis and psychotic disorders among problem users also rose.

The commercialisation divide

Researchers attribute the divergence to market structure. In Europe, Africa, and Oceania, policy changes have been smaller and more restricted, and no direct links were found between those steps toward legalisation and shifts in use or disorder rates. The team points to the US and Canada as the clearest examples of commercialised models, where regional differences (state-level rules in the US, provincial retail rules in Canada) still shape outcomes.

Germany's test case

Germany partially legalised cannabis in April 2024. Adults may cultivate up to three plants at home, possess up to 50 grams at home and 25 grams in public, and join non-commercial cultivation associations of up to 500 members.

As of 2025, we were unable to detect significant changes in cannabis consumption in Germany since legalisation.

The short observation period demands continued monitoring, but the low degree of commercialisation could limit future consumption shifts.

Policy implications

Because cannabis policies are evolving quickly worldwide, the study's authors stress the need to evaluate how different legal frameworks influence public health. The contrast between controlled, non-commercial approaches and open retail markets suggests that the design of legalisation matters at least as much as the decision to legalise itself.