Elevated rates of cannabis use have repeatedly been observed among individuals with schizophrenia. Although several studies have investigated the role of cannabis exposure in the onset of psychosis, the clinical presentation and the course of the illness, the effects have been inconsistent.

To assess differences in the clinical characteristics of schizophrenic patients with and without cannabis abuse.

To identify the clinical and therapeutic features of schizophrenic patients who abuse cannabis.

This is a case-control cross-sectional study of schizophrenic patients with either no history of cannabis abuse (NCA; N = 30) or a history of cannabis abuse (CA; N = 30). We used a semi-structured interview, the global assessment of functioning scale (GAF) and the Mini mental State examination (MMSE) to assess the two groups for psychopathologic symptoms and cognitive functioning.

Compared to the NCA group, the CA group demonstrated significantly worse performance on measures of MMSE (25,73 vs 26,6, p = 0,034) and GAF (58,5 vs 66,7, p = 0,001). The CA group was also significantly associated with an increase in antipsychotics side effects (p = 0,01). Only 46,7% of patients with a history of cannabis abuse was adherent to medication while 76,7% of patients who did not use cannabis took their treatment regularly. The difference between the two groups was statically significant (p = 0,019).

As our understanding of the biology of cannabis use and psychosis continues to improve, it may be possible to identify mechanisms by which exposure to cannabis may influence psychosis that may become future targets for intervention.