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

This review examines two hypotheses linking cannabis use to psychosis. The first, that heavy cannabis use can cause a distinct 'cannabis psychosis' that would not otherwise occur, has limited clinical support; such cases appear rare and likely require very high tetrahydrocannabinol doses, prolonged use of potent cannabis, or a preexisting vulnerability. The second hypothesis, that cannabis use may precipitate or worsen schizophrenia, has more backing: one large prospective study found a linear relationship between cannabis use frequency by age 18 and schizophrenia diagnosis risk over 15 years. However, it remains unclear whether cannabis precipitates schizophrenia, represents self-medication, or if other drug use explains the association. Cannabis use can exacerbate schizophrenia symptoms.

Study at a glance

Abstract

This paper evaluates evidence for two hypotheses about the relationship between cannabis use and psychosis: (i) that heavy cannabis use causes a 'cannabis psychosis', i.e. a psychotic disorder that would not have occurred in the absence of cannabis use and which can be recognised by its pattern of symptoms and their relationship to cannabis use; and (ii) that cannabis use may precipitate schizophrenia, or exacerbate its symptoms. Literature relevant to drug use and schizophrenia is reviewed. There is limited clinical evidence for the first hypothesis. If 'cannabis psychoses' exist, they seem to be rare, because they require very high doses of tetrahydrocannabinol, the prolonged use of highly potent forms of cannabis, or a preexisting (but as yet unspecified) vulnerability, or both. There is more support for the second hypothesis in that a large prospective study has shown a linear relationship between the frequency with which cannabis had been used by age 18 and the risk over the subsequent 15 years of receiving a diagnosis of schizophrenia. It is still unclear whether this means that cannabis use precipitates schizophrenia, whether cannabis use is a form of 'self-medication', or whether the association is due to the use of other drugs, such as amphetamines, 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.