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Tijdschrift voor Psychiatrie 46 (2004) 8, 515 - 524

Review article

Cannabis as a risk factor for schizophrenia

J.G.C. van Amsterdam, A. Opperhuizen, W. van den Brink

background According to some recent reports in the literature, chronic use of cannabis induces schizophrenia. The first symptoms of schizophrenia often appear during adolescence, which is the time when youngsters often start experimenting with cannabis. Not surprisingly, therefore, some investigators see a clear link between the use of cannabis and the development of schizophrenia.
aim To investigate whether there's a causal relationship between the use of cannabis and schizophrenia.
method Medline was searched using winspirrs and using as key words 'cannabis-', 'marijuana abuse-', in combination with 'schizophrenia-'; 'psychotic-disorders'; 'psychoses-substance-induced'.
results Although the available literature clearly indicates a link between schizophrenia and the use of cannabis there is no evidence of a causal relationship. There are so many confounding factors and investigations aimed at establishing a relationship will need to take these confounders into account. In all the recently reviewed studies that have attempted to correct for such confounders the strength of the observed relationship weakened and in some cases the relationship was no longer statistically significant. Finally, it is remarkable that the steep increase in the use of cannabis in the general population does not seem to have led to an increase in the incidence of schizophrenia. Presumably, in vulnerable subjects who use cannabis a schizophrenic episode will probably occur sooner and chronic or intense use of cannabis can negatively affect the course of schizophrenia. It is therefore advisable that youngsters with a family history of schizophrenia and patients with a schizophrenic disorder be discouraged from using cannabis.
conclusion On the basis of these considerations it must be concluded that there is no conclusive evidence for a causal relationship between the use of cannabis and the development of schizophrenia and that evidence for such a relationship is unlikely to be produced in the future either. Furthermore, on the grounds of the data that are currently available there is no justification for the proposed closure of coffee shops.

keywords cannabis, psychosis, schizophrenia