Objective: Our aim was to review recent studies and estimate the rate of cannabis use disorders (CUDs) in schizophrenia, as well as to examine the factors affecting this rate.
Methods: We conducted an electronic search of 3 literature databases and a manual search of articles from 1996 to 2008. The key words used were "schizophreni*," "psychos*s," "psychotic," "cannabis abuse," "cannabis dependence," "cannabis use disorder," "substance use disorder," "substance abuse," "substance dependence," and "dual diagnosis." Articles that reported diagnoses according to the Diagnostic and Statistical Manual of Mental Disorders or International Classification of Diseases were included. Regression analysis was used to examine how estimated rates of CUDs are affected by various study characteristics such as the classification system, inpatient vs outpatient status, study location, proportion of males, age of the sample, or duration of illness.
Results: Thirty-five studies met our search criteria. The median current rate of CUDs was 16.0% (interquartile range [IQR] = 8.6-28.6, 10 studies), and the median lifetime rate was 27.1% (IQR = 12.2-38.5, 28 studies). The median rate of CUDs was markedly higher in first-episode vs long-term patients (current 28.6%/22.0%, lifetime 44.4%/12.2%, respectively) and in studies where more than two-thirds of the participants were males than in the other studies (33.8%/13.2%). CUDs were also more common in younger samples than in the others (current 38.5%/16.0%, lifetime 45.0%/17.9%).
Conclusions: Approximately every fourth schizophrenia patient in our sample of studies had a diagnosis of CUDs. CUDs were especially common in younger and first-episode patient samples as well as in samples with a high proportion of males.