Does cannabis use cause schizophrenia? The epidemiological evidence

In this chapter we examine the strength of the epidemiological evidence to support a causal relationship between cannabis use and increased risk of schizophrenia and other psychotic disorders. We discuss the implications of this for individuals who use cannabis and for public health at a population level. The reader is also referred to Chapters 5 and 12 of this volume. There is little dispute that cannabis use can cause short-lived and mild psychotic experiences (hallucinations, delusions and periods of thought disorder) directly following use. Such adverse effects are commonly reported in surveys of users and have support from experimental studies of the effects of Δ9-tetrahydrocannabinol (THC) in humans, too (see Chapter 12). It is likely that the transient nature of such symptoms cause minimal functional impairment in most individuals. Of much greater concern, however, is whether cannabis use increases the risk of more severe and prolonged psychotic states that persist beyond the period reasonably attributable to direct biological effects of exogenous cannabinoids. Whether such a relationship exists is often disputed in the literature, but it is important to establish the truth given that these disorders lead to substantial distress to individuals and their families and to public burden from healthcare costs.