Dual diagnosis of schizophrenia and substance-related disorders is common in psychiatric practice. Epidemiologic studies and report have established that the risk of a substance-related disorder was 4 to 5 times higher in a population of psychiatric patients than in the general population. However, little is known on the reason of this relationship and the treatments required. It's well known that a family history of psychosis is a risk factor of schizophrenia. Similarly a family history of substance use disorders increases the risk of using substances. Because the two disorders often occurred together, it could be hypothesized that a genetic risk factor is common. However, recent studies did not confirm this hypothesis and it seems that their genetic risks factor would be unrelated. Evidence now exists describing the different profiles of patients whether they used substance or not. Concerning negative symptoms clinical studies and meta-analyses have described fewer symptoms in schizophrenia patients with a substance use disorder. Among the different explanations that have been addressed, it seems that a lower capability of obtaining the substance could partly explain this relationship. Perhaps because patients with social withdrawal have more difficulties to find and spend the time required to obtain abused substances. At the opposite some products such as cocaine may relieve some symptoms especially anhedonia and alogia. However the link between substance-related disorders and negative symptoms is weak and decreases in more recent studies, probably because negative symptoms as well as addiction disorders are better characterized. Considering that treating psychiatric symptoms may not always lead to a decrease in the substance-related disorders but that patients who give up substances improve their psychotic symptoms, a therapeutic strategy should be planned for these dual disorders patients combining psychiatry and addiction interventions.