Morbid risk of schizophrenia for relatives of patients with cannabis-associated psychosis.

Twenty-three patients admitted with acute psychosis who were cannabis positive on urinary screening were each matched, with respect to sex, with two psychotic controls who screened negatively for all substances. The lifetime morbid risk of psychiatric disorder was estimated among the first degree relatives of cases and controls, using RDC-FH criteria to define diagnoses, and Weinberg's shorter method of age correction. The cases had a significantly greater familial morbid risk of schizophrenia (7.1%) than the controls (0.7%), while the risks of other psychoses, and of non-psychotic conditions were similar. The same pattern of familial risk was evident when the analysis was restricted to patients with DSM-III schizophrenia. The data suggest that the development or recurrence of acute psychosis in the context of cannabis use may be associated with a genetic predisposition to schizophrenia.