There are a number of good reasons why it is a plausible hypothesis that cannabis use may be a cause of psychotic disorders, severe mental illnesses in which hallucinations, delusions and impaired reality testing are predominant features. First, THC is a psychoactive substance which produces some symptoms found in psychotic disorders: euphoria, distorted time perception, cognitive and memory impairments (Thornicroft, 1990). Second, in laboratory studies high doses of THC have produced psychotic symptoms in normal volunteers, including visual and auditory hallucinations, delusional ideas, thought disorder, and symptoms of hypomania (Georgotas and Zeidenberg, 1979). Third, clinical observers in countries which have a long history of chronic cannabis use, such as India, Egypt, and the Caribbean, have described 'cannabis psychoses’ (Brill and Nahas, 1984; Ghodse, 1986).

It is useful to distinguish three types of hypotheses about relationships between cannabis use and psychosis (Hall, 1998). The strongest hypothesis is that heavy cannabis use causes a unique ‘cannabis psychosis’. This hypothesis assumes (1) that the psychosis would not occur in the absence of cannabis use, and (2) that the causal role of cannabis can be inferred from the type of psychotic symptoms and the observations that psychotic symptoms are preceded by heavy cannabis use and remit with abstinence.

A second hypothesis is that cannabis use may precipitate an episode of schizophrenia. This hypothesis assumes that cannabis use is one factor among many others (including genetic predisposition and other unknown causes) that act together to cause schizophrenia. It does not assume that a causal role for cannabis can be inferred from the symptoms of the disorder, or that the disorder will necessarily remit when cannabis use ceases.

Third, if cannabis use can precipitate schizophrenia, it is also probable that it can exacerbate its symptoms. However, cannabis use may exacerbate symptoms of schizophrenia (even if it is not a precipitant of the disorder) by reducing compliance with treatment, or interfering with the effects of the neuroleptic drugs that are used to treat it.

Making causal inferences

In order to infer that cannabis use is a cause of psychosis in any of these ways we need evidence: that there is an association between cannabis use and psychosis; that chance is an unlikely explanation of the association; that cannabis use preceded the psychosis; and that plausible alternative explanations of the association can be excluded (Hall, 1987).