Objective: To study the role of cannabis use in the onset of symptoms and disorders in the schizophrenia spectrum.
Design: Literature study.
Method: Hypothetical explanations of the relationship between cannabis use and subsequent schizophrenia were assessed on the basis of the results of five large longitudinal studies.
Results: Because cannabis use preceded the development of schizophrenia and as a result of statistical control for possible confounders, the following explanations could be rejected: 'cannabis is used as self-medication for schizophrenia', 'schizophrenia is not caused by cannabis but by other drugs that are used concurrently', and 'cannabis use and schizophrenia are both caused by other factors'. Two explanations then remained: 'cannabis use per se contributes in a unique manner to the risk' and 'cannabis use, in interaction with other risk factors, leads to an increase in the risk'.
Conclusion: There are strong indications that cannabis use increases the risk of the subsequent development of symptoms and disorders in the schizophrenia spectrum.