"This 10 year follow-up study showed that incident cannabis use significantly increased the risk of incident psychotic experiences. The association was independent of age, sex, socioeconomic status, use of other drugs, urban/rural environment, and childhood trauma; additional adjustment for other psychiatric diagnoses similarly did not change the results. There was no evidence for self medication effects as psychotic experiences did not predict later cannabis use. The results thus help to clarify the temporal association between cannabis use and psychotic experiences by systematically addressing the issue of reverse causality, given that the long follow-up period allowed exclusion of all individuals with pre-existing psychotic experiences or pre-existing cannabis use. In addition, cannabis use was confirmed as an environmental risk factor impacting on the risk of persistence of psychotic experiences (fig 3)."
Keupper, Rebecca, van Os, Jim, et al., "Continued Cannabis Use and Risk of Incidence and Persistence of Psychotic Symptoms: 10-Year Follow-Up Cohort Study, British Medical Journal, 2011;342:d738 doi:10.1136/bmj.d738