The researchers analyzed anonymized routine data on inpatient treatment cases from the six hospitals of the Bezirkskliniken Schwaben, which serve the entire administrative district with a population of around 1.9 million. They compared the two years prior to legalization (from April 2022) with the first year after legalization (April 2024 to March 2025). It is the first study to investigate the suspected link in a German region.
The results show that mental health problems caused by cannabis use increased by a factor of 1.5 during the period studied. Cannabis-induced psychoses even increased by almost a factor of two. The total number of inpatient treatment cases, however, remained stable.
Strengthening targeted prevention
“Whether the increase in cannabis-related problems is actually causally related to legalization cannot be proven with certainty based on this data,” says Prof. Dr. Alkomiet Hasan, Chair of Psychiatry and Psychotherapy at the Medical Faculty of the University of Augsburg and co-author of the study.
Other factors could also play a role in the observed increase. For example, increased diagnostic attention, greater openness among those affected due to reduced stigmatization, and the expectations of the medical profession may have influenced the measurements. Longer observation periods are necessary to make reliable assessments.
“The results should prompt us to closely monitor developments scientifically and to invest more in prevention now,” said study leader Dr. Sophie-Kathrin Greiner. In addition to general information campaigns about the risks of cannabis use, it is important to specifically educate vulnerable groups—young people and people with mental illness—about the risk of psychosis.
Publication:
Häufigkeit psychotischer Erkrankungen nach der Cannabis-Legalisierung
, Deutsches Ärzteblatt
