Cannabis is the most widely used illegal drug in Spain. Currently, its use is on the rise as risk perception is decreasing, primarily among young people. It is well known that cannabis negatively influences course and prognosis in schizophrenic patients. However, the relationship between cannabis use and development of a psychotic or schizophrenic disorder remains controversial. The study of this topic has been approached using longitudinal cohort studies, which study cannabis use and psychotic or schizophrenic disorders. In addition to the classic Swedish conscript study published by Andreasson et al. 1987, during the past years, six more longitudinal cohort studies have been published. The data demonstrate that there are both temporal and dose-response relationships, and that early initiation of cannabis use is highly correlated with the development of psychotic symptoms. Cannabis consumption can increase the risk of developing schizophrenia in a vulnerable population twofold, to the extent that some studies suggest a causal relationship. The current knowledge base makes it necessary to warn the population about the relationship between cannabis use and the development of psychosis.