These statistical correlation studies only show the mentally ill are more likely to self medicate and become addicts. Twin studies have shown no measurable differences among twins from even the heaviest cannabis abuse.
Alcohol abuse was actually shown to create direct structural changes in the brain in this twin study while cannabis abuse did not. https://pubmed.ncbi.nlm.nih.gov/33726938/
The correlation of mental illness to tobacco is actually FAR stronger than cannabis yet few are claiming it MAY cause mental illness like these reefer madness rags. 80% of schizos smoke cigarettes versus 12% of general population. Far greater than this 30% cited for cannabis.
The study:
“The Effects of Alcohol and Cannabis Use on the Cortical Thickness of Cognitive Control and Salience Brain Networks in Emerging Adulthood: A Co-twin Control Study”
Does not mention schizophrenia in any way.
Here’s a study on that topic:
Cannabis use and risk of schizophrenia: a Mendelian randomization study
...”This adds to the substantial evidence base that has previously identified cannabis use to associate with increased risk of schizophrenia, by suggesting that the relationship is causal. Such robust evidence may inform public health messages about cannabis use, especially regarding its potential mental health consequences.”
https://pubmed.ncbi.nlm.nih.gov/28115737/