Cannabinoid use and schizophrenia are correlated, but there isn't proven causation. The observation is people who use cannabinoid have an increased likelihood of having schizophrenia. So it could be that people who are going to develop schizophrenia also have the biology that predisposed to cannabinoid use, or that cannabinoid use causes schizophrenia. It's challenging to tease this out with ethical studies. Given recent legislation that marijuana is more available, it's possible we see an increase in schizophrenia which would given credence to the causation theory.

--- Nikhil Dhawan, MD 2020/07/16 16:35

https:pro.psychcentral.com/cannabis-induced-psychosis-on-the-rise-in-teens/ Pubmed abstract Am J Psychiatry . 2018 Apr 1;175(4):343-350. doi: 10.1176/appi.ajp.2017.17020223. Epub 2017 Nov 28. Rates and Predictors of Conversion to Schizophrenia or Bipolar Disorder Following Substance-Induced Psychosis Marie Stefanie Kejser Starzer 1, Merete Nordentoft 1, Carsten Hjorthøj 1 Affiliations expand PMID: 29179576 DOI: 10.1176/appi.ajp.2017.17020223 Erratum in CORRECTION. Am J Psychiatry. 2019 Apr 1;176(4):324. doi: 10.1176/appi.ajp.2019.1764correction2. PMID: 30929503 No abstract available. Abstract Objective: The authors investigated the rates of conversion to schizophrenia and bipolar disorder after a substance-induced psychosis, as well as risk factors for conversion. Method: All patient information was extracted from the Danish Civil Registration System and the Psychiatric Central Research Register. The study population included all persons who received a diagnosis of substance-induced psychosis between 1994 and 2014 (N=6,788); patients were followed until first occurrence of schizophrenia or bipolar disorder or until death, emigration, or August 2014. The Kaplan-Meier method was used to obtain cumulative probabilities for the conversion from a substance-induced psychosis to schizophrenia or bipolar disorder. Cox proportional hazards regression models were used to calculate hazard ratios for all covariates. Results: Overall, 32.2% (95% CI=29.7-34.9) of patients with a substance-induced psychosis converted to either bipolar or schizophrenia-spectrum disorders. The highest conversion rate was found for cannabis-induced psychosis, with 47.4% (95% CI=42.7-52.3) converting to either schizophrenia or bipolar disorder. Young age was associated with a higher risk of converting to schizophrenia. Self-harm after a substance-induced psychosis was significantly linked to a higher risk of converting to both schizophrenia and bipolar disorder. Half the cases of conversion to schizophrenia occurred within 3.1 years after a substance-induced psychosis, and half the cases of conversion to bipolar disorder occurred within 4.4 years. Conclusions: Substance-induced psychosis is strongly associated with the development of severe mental illness, and a long follow-up period is needed to identify the majority of cases. Keywords: Alcohol Abuse; Mood Disorders-Bipolar; Psychoactive Substance Use Disorder; Psychosis; Schizophrenia.