Objective: The authors investigated the rates of conversion to schizophrenia and bipolar disorder after a substance-induced psychosis, as well as risk factors for conversion.

Method: All patient information was extracted from the Danish Civil Registration System and the Psychiatric Central Research Register. The study population included all persons who received a diagnosis of substance-induced psychosis between 1994 and 2014 (N=6,788); patients were followed until first occurrence of schizophrenia or bipolar disorder or until death, emigration, or August 2014. The Kaplan-Meier method was used to obtain cumulative probabilities for the conversion from a substance-induced psychosis to schizophrenia or bipolar disorder. Cox proportional hazards regression models were used to calculate hazard ratios for all covariates.

Results: Overall, 32.2% (95% CI=29.7-34.9) of patients with a substance-induced psychosis converted to either bipolar or schizophrenia-spectrum disorders. The highest conversion rate was found for cannabis-induced psychosis, with 47.4% (95% CI=42.7-52.3) converting to either schizophrenia or bipolar disorder. Young age was associated with a higher risk of converting to schizophrenia. Self-harm after a substance-induced psychosis was significantly linked to a higher risk of converting to both schizophrenia and bipolar disorder. Half the cases of conversion to schizophrenia occurred within 3.1 years after a substance-induced psychosis, and half the cases of conversion to bipolar disorder occurred within 4.4 years.

Conclusions: Substance-induced psychosis is strongly associated with the development of severe mental illness, and a long follow-up period is needed to identify the majority of cases.