Epidemiological studies suggest that 20 % to 50 % of patients with schizophrenia have a lifetime comorbid substance use disorder (SUD). In first-episode psychosis this prevalence is even higher and varies between 20 % and 75 % with cannabis being the most widely used illicit drug. These difficult to treat patients usually have a worse prognosis as compared with non-substance abusing schizophrenic patients. Despite multiple theories proposed such as the self medication hypothesis, common or bidirectional factor models or genetic vulnerability, there is no consensus on the aetiology of increased rates of substance use in people with psychosis which is important to treat these patients. The dually diagnosed population is a heterogeneous group and it is likely that different models may explain comorbidity in different subgroups. The present review part one gives an overview on prevalence and explanation models for dual diagnosis psychosis and substance use with focus in adolescent and young adult populations, the second part reviews the clinical course for both disorders and current psychosocial treatment options.
