Moore and colleagues9 … found that individuals who had used cannabis had a significant, 1.4-fold increased risk of any psychotic outcomes, independent of potential confounding and transient intoxication effects. Findings also provided evidence for a dose-response effect, with even greater, 2.1-fold risk in individuals who used cannabis most frequently.
More recently, Marconi and colleagues10 performed a meta-analysis of 10 studies, including 66,810 individuals, that investigated the association between the degree of cannabis consumption and risk of psychosis. In all individual studies, higher levels of cannabis use were associated with increased risk of psychosis. They also found evidence for a dose-response relationship, with a 2-fold increase in risk for the average cannabis user, and a 4-fold increase in risk for the heaviest users, compared with non-users.