“The study cohort comprised almost 600,000 patients each year, representing approximately 2.3% of the UK population aged 16 to 44. Between 1996 and 2005 the incidence and prevalence of schizophrenia and psychoses were either stable or declining. Explanations other than a genuine stability or decline were considered, but appeared less plausible. In conclusion, this study did not find any evidence of increasing schizophrenia or psychoses in the general population from 1996 to 2005.”
https://www.ncbi.nlm.nih.gov/pubmed/19560900
https://www.sciencedirect.com/science/article/pii/S0920996409002692?via%3Dihub
“The incidence and prevalence of patients showing schizophrenic syndromes are unchanged or have even fallen while the use of cannabis has increased enormously. We must conclude that either previous schizophrenic illnesses have become much less common or that cannabis schizophrenia is rare and perhaps it may not even exist.” https://www.bmj.com/content/325/7374/1183/rapid-responses
“Cannabis use appears to be neither a sufficient nor a necessary cause for psychosis.” https://www.ncbi.nlm.nih.gov/pubmed/14754822
“The current data do not support low to moderate lifetime cannabis use to be a major contributor to psychosis or poor social and role functioning in high-risk youth.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3459073/