There are good reasons to be concerned about the possibility that cannabis use may be a cause of psychotic disorders. Psychoses are serious and disabling disorders (Bromet et al., 1995). Cannabis is widely used during late adolescence in many developed societies (Hall et al., 1999), and high doses of tetrahydrocannabinol (THC) – the psychoactive substance in cannabis – have been reported to produce psychotic symptoms such as visual and auditory hallucinations, delusional ideas and thought disorder in normal volunteers (Hall et al., 2001; and see Chapter 5).

There are a number of hypotheses about the relationship between cannabis use and psychosis that need to be distinguished (Thornicroft, 1990). The strongest hypothesis in causal terms is that heavy cannabis use causes a specific ‘cannabis psychosis’. It assumes that these psychoses would not occur in the absence of cannabis use, and that the causal role of cannabis can be inferred from the symptoms and their relationship to cannabis use; that is, they are preceded by heavy cannabis use and remit after abstinence. It also assumes that cannabis psychoses are qualitatively different from other psychotic disorders. This hypothesis is the subject of this chapter. The potential role of cannabis as a causal agent for schizophrenia per se is the subject of Chapter 7.

Making causal inferences

In order to infer that cannabis use is a cause of a specific psychotic disorder, 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).