Up to 50% of cases of cannabis-induced psychosis covert to schizophrenia. A new study provides striking evidence for abuse of other substances and schizophrenia onset.
• One hypothesis for the comorbidity between psychiatric disorders and substance use disorders is that patients use substances as a means to “self-medicate”
• However, cannabis use may accelerate the onset of schizophrenia in vulnerable individuals in a dose-dependent manner 
• Many substances themselves can cause psychosis
• Up to 50% of cases of cannabis-induced psychosis covert to schizophrenia; less is known about other substances
• The presence of a substance use disorder may delay the diagnosis of a comorbid psychiatric disorder, and therefore treatment
• Starzer and colleagues  investigated the proportion of patients diagnosed with substance-induced psychosis who later developed schizophrenia or bipolar disorder, and risk factors for the conversion of substance-induced psychosis to these diagnoses
• The most common substances were alcohol (34%) and cannabis (22%); other substances included opioids, sedative, cocaine, amphetamines, hallucinogens, and mixed/other
• The 20-year conversion rate to schizophrenia was 26%
• The highest substance-specific conversion rate was 41% for cannabis-induced psychosis to schizophrenia (32% for amphetamines; 28% for hallucinogens; 22% for opioids; 21% for sedatives; and 20% for cocaine)
• Subjects with substance-induced psychosis were >77 times more likely than comparison subjects to convert to schizophrenia
• Half of the conversions to schizophrenia occurred within 3 years
• The risk of conversion to schizophrenia decreased with increasing age at incident substance-induced psychosis
• Personality disorders and eating disorders, as well as self-harm after a substance-induced psychosis, were significantly linked to conversion to schizophrenia
• The authors concluded that greater than 1 in 4 patients with substance-induced psychosis converted to schizophrenia; with half of the conversions occurring within 3 years
• The highest conversion rate was for cannabis-induced psychosis
• The authors note the register-based cohort design as both a strength and limitation of the study
Take Home Point
• All patients with substance-induced psychosis should be offered follow-up for at least 2 years, especially those who present with self-harm
1. Aresenault L, Cannon M, Poulton R, et al. Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ. 2002;35:1212-1213.
2. Arendt M, Rosenberg R, Foldager L, et al. Cannabis-induced psychosis and subsequent schizophrenia-spectrum disorders: follow-up study of 535 incident cases. Br J Psychiatry. 2005;187:510-515.
3. Starzer MSK, Nordentoft M, Hjorthoj C. Rates and predictors of conversion to schizophrenia following substance-induced psychosis. Am J Psychiatry. 2018;175:343-350.