Cannabis use disorder is associated with an earlier age at onset and a more severe outcome of schizophrenia spectrum disorders. The role of cannabis use before the onset of illness (premorbid cannabis use) has not been fully investigated. We here examined how amount and type of premorbid cannabis use was associated with the later course of illness including current substance use, symptoms and level of functioning in schizophrenia spectrum disorder.
We used a naturalistic sample of patients with DSM-IV schizophrenia spectrum disorders with a comprehensive history of illness and substance use. Data on premorbid substance use was obtained from comprehensive self-report. The relationship to outcome was investigated using regression models that included current substance use and premorbid functioning.
Pre-schizophrenia cannabis use was significantly associated with more severe psychotic symptoms and impaired functioning. Higher levels of premorbid cannabis use were associated with higher levels of current psychotic symptoms. These associations were independent of current substance use and premorbid functioning. Early use of cannabis (age <17 years) was associated with earlier age at onset of psychosis, independently of potential confounders.
Pre-psychosis cannabis use affects illness outcome in schizophrenia spectrum disorders, and is associated with lower age at onset of psychosis. These findings of independent negative effects of premorbid cannabis use in schizophrenia suggest that a limitation of the general use of cannabis may have beneficial health effects.