Study shows an increase in cases of psychosis following the legalisation of cannabis

The legalisation of cannabis could be responsible for more cases of psychosis and hospital admissions. This is suggested by a pilot study conducted at the Chair and Clinic for Psychiatry, Psychotherapy, and Psychosomatics at the University of Augsburg, led by Prof. Alkomiet Hasan. The study published in the German medical journal Ärzteblatt examined the development of hospitalisation rates and cases of cannabis-induced psychosis in the period around legalisation in the Swabian region of Bavaria and observed a relevant increase. The possession and consumption of cannabis for recreational purposes was legalised in Germany on the 1st of April 2024.

Following the legalisation of cannabis more users are suffering from psychosis or mental health problems than previously. This is indicated by a study presented by researchers from the Faculty of Medicine at the University of Augsburg and Swabia’s district clinics in a short report published in the German medical journal Ärtzeblatt.

They ascertained how many adults in the two years prior to legalisation (from April 2022) and in the year following legalisation (from April 2024 to the end of March 2025) were treated in Swabia’s district clinics for cannabis-related problems.

Mental health problems generally caused by the consumption of cannabis increased by one and a half times following legalisation. The number of cannabis-induced psychoses even nearly doubled. However, the total number of inpatient treatment cases remained relatively stable during the study period.

First data in Germany

The six hospitals belonging to the district clinics cover psychiatric hospital care for the entire administrative district of Swabia, which has a population of around 1.9 million and includes the City of Augsburg. The pilot study was conducted as part of the activities of the Deutsches Zentrums für Psychische Gesundheit (DZPG) (German Centre for Mental Health) and is the first to investigate the previously only suspected connection between the consumption of cannabis and mental health problems in a German region. The analyses were conducted anonymously and included routine data on inpatient treatment cases.

Further research and prevention important

“Whether the increase in cannabis-related problems is actually causally linked to legalisation cannot be proven with certainty based on this data,” explains Hasan, Chair of Psychiatry and Psychotherapy at the Faculty of Medicine at the University of Augsburg and co-author of the study. “However, the results should prompt us to scientifically closely monitor developments and to invest more in prevention now,” says Dr. Sophie-Kathrin Greiner, who led the study.

The shift could also be due to increased diagnostic attention and greater openness on the part of those affected as a result of reduced stigmatisation. The measurements in the study could also have been influenced by the fact that more cannabis-induced psychoses were expected by doctors and therefore more were diagnosed. Longer observation periods would also be necessary to reliably estimate the conversion rates of cannabis-induced psychoses and psychotic disorders.

The possible increase in cannabis-induced psychoses could speak in favour of more targeted prevention measures. In addition to general information campaigns about the risks of cannabis use, particularly vulnerable groups (young people, people with mental illness) should be educated about the risk of psychosis, say the authors of the study.