Cannabis use and psychosis: a review of clinical and epidemiological evidence

Objective: This paper evaluates evidence for two hypotheses about the relat ionship between cannabis use and psychosis: (i) that heavy cannabis use cau ses a 'cannabis psychosis', i.e, a psychotic disorder that would not have o ccurred in the absence of cannabis use and which can be recognised by its p attern of symptoms and their relationship to cannabis use; and (ii) that ca nnabis use may precipitate schizophrenia, or exacerbate its symptoms. Method: Literature relevant to drug use and schizophrenia is reviewed. Results: There is limited clinical evidence for the first hypothesis. If 'c annabis psychoses' exist, they seem to be rare, because they require very h igh doses of tetrahydrocannabinol, the prolonged use of highly potent forms of cannabis, or a preexisting (but as yet unspecified) vulnerability, or b oth. There is more support for the second hypothesis in that a large prospe ctive study has shown a linear relationship between the frequency with whic h cannabis had been used by age 18 and the risk over the subsequent 15 year s of receiving a diagnosis of schizophrenia. Conclusions: It is still unclear whether this means that cannabis use preci pitates schizophrenia, whether cannabis use is a form of 'self-medication', or whether the association is due to the use of other drugs, such as amphe tamines, which heavy cannabis users are more likely to use. There is better clinical and epidemiological evidence that cannabis use can exacerbate the symptoms of schizophrenia.