Cannabis use is linked to treatment non-adherence and relapses in psychotic disorders. Antipsychotic medication is effective for relapse prevention in primary psychoses, but its effectiveness after cannabis-induced psychosis (CIP) remains unclear.

To examine the effectiveness of antipsychotic medication for relapse prevention following the first clinically diagnosed CIP.

A cohort of 1772 patients (84.1% men) with incident CIP was identified from the Swedish National Patient and Micro Data for Analyses of Social Insurance registers. The primary outcome was hospitalisation due to any psychotic episode. Drug use data were collected from the Prescribed Drug Register and modelled into drug use periods using the PRE2DUP method. A within-individual Cox regression model was used to study the risk of outcomes during the use of different oral or long-acting injectable (LAI) antipsychotics compared with non-use.

The mean age at first diagnosis was 26.6 years (s.d. = 8.3). Of the cohort, 1343 (75.8%) used antipsychotics and 914 (51.3%) experienced psychosis hospitalisation during the follow-up. Any antipsychotic use was associated with a decreased risk of psychosis hospitalisation (adjusted hazard ratio (aHR) 0.75; 95% CI 0.67–0.84). Specific antipsychotics associated with decreased risk included aripiprazole LAI (aHR 0.27; 95% CI 0.14–0.51), olanzapine LAI (aHR 0.28; 95% CI 0.15–0.53), clozapine (aHR 0.55; 95% CI 0.34–0.90), oral aripiprazole (aHR 0.64; 95% CI 0.45–0.91), antipsychotic polytherapy (aHR 0.74; 95% CI 0.63–0.87) and oral olanzapine (aHR 0.81; 95% CI 0.69–0.94).

In particular, LAIs, clozapine and oral aripiprazole were associated with a decreased risk of psychosis relapse following CIP. Prescribers should consider using more LAIs for better treatment outcomes after CIP.

Cannabis-induced psychotic disorder (CIPD) is defined by the development of psychotic symptoms during or briefly after intoxication with cannabis or withdrawal from cannabis. The social measures and restrictions implemented following the COVID-19 pandemic might have had an impact on cannabis availability, as suggested by patients from our clinical practice, reporting a shortage of the substance.

To compare sociodemographic, clinical characteristics and admission rates of inpatient treatment for cannabis-induced psychotic disorder in COVID-19 pandemic period and pre-pandemic period.

Retrospective observational study of inpatient admissions for CIPD in a psychiatry inpatient unit of a tertiary hospital. The statistical analysis was performed using SPSS software, version 27.0.

Our sample included 120 inpatient admissions, corresponding to 80 patients. Compared to 2018 and 2019, in 2020 there was an overall reduction of 21.5% in inpatient admissions (n=618, 549 and 458, respectively). The number of admissions for CIPD in 2018, 2019, 2020 and 2021 up to september were, respectively, 29, 32, 10 and 31 (5.2%, 6.1%, 2.2% and 7.2% of respective annual admissions). We found no statistically significant differences regarding sociodemographic and clinical characteristics in patients admitted for CIPD during 2020.

These results suggest a disproportionate reduction of inpatient admissions due to CIPD in 2020, followed by an expressive increase in the number of admissions in 2021up to september. This might be related to cannabis availability returning to regular levels. However other factors must be considered, such as the delay of treatment due to reduced accessibility to health care.

No significant relationships.

Worldwide, cannabis is the most used illegal substance, and the use of cannabis has increased over the years. An increase in the level of tetrahydrocannabinol (THC) in cannabis has also been seen. It is currently unclear whether this has led to an increase in the incidence of cannabis-induced psychosis. We aimed to investigate (1) the development of incidence of cannabis-induced psychosis over time compared with other substance-induced psychoses and (2) the development of incident cases of cannabis-induced psychosis over time compared with dual diagnosis defined as schizophrenia and a cannabis use disorder.

Data on psychiatric diagnoses were extracted from the Danish Psychiatric Central Research Register and summarized per year as both absolute incidence (number of cases) and incidence rates per 100 000 person years.

The incidence rate of cannabis-induced psychosis increased steadily from 2.8 per 100 000 person years in 2006 to 6.1 per 100 000 person years in 2016. There was a corresponding increase in dual diagnosis with schizophrenia and cannabis use disorder, but a decrease in alcohol-induced psychosis. The data showed no trend in the other substance-induced psychosis investigated in this thesis.

The increase in cannabis-induced psychosis follows both the increase in the level of THC in cannabis, and the increase in cannabis use. The change in diagnostic practice does not appear to explain the increase in incidence of cannabis-induced psychosis.