This section reviews evidence on three adverse psychological effects that have been attributed to chronic cannabis use. These are: the effects of regular cannabis use on motivation; the effects of chronic regular use on cognitive functioning; and the contribution that cannabis use might make to the onset and exacerbation of psychotic disorders, including schizophrenia.

A number of common issues emerge in considering these three types of effect. First, all three concerns have been prompted by case histories of individuals whose motivation or cognitive performance had been adversely affected, or who exhibited psychotic symptoms after using cannabis. Second, as we will point out, case histories have a limited capacity to inform us about whether cannabis use plays a causal role in these disorders. In each case, a plausible alternative explanation that has to be excluded is that the relationship arises because people who are most likely to use cannabis are at higher risk of impaired motivation, a preoccupation with cannabis use, cognitive impairment and psychosis, before they used cannabis. The causal criteria outlined in the introduction to Section 2 are critical in sorting out cause and effect. Third, the question of whether cannabis does play a causal role in each of these disorders is important because all of the disorders can have serious adverse consequences for the persons affected and their families.