https://nypost.com/2019/08/07/the-link-between-pot-and-mass-shootings-may-be-closer-than-we-think/
In 2007 the prestigious medical journal Lancet recanted its previous benign view of marijuana, citing studies showing “an increase in risk of psychosis of about 40 percent.”
A seminal long-term study of 50,465 Swedish army conscripts found those who had tried marijuana by age 18 had 2.4 times the risk of being diagnosed with schizophrenia in the following 15 years than those who had never used the drug. Heavy users were 6.7 times more likely to be admitted to a hospital for schizophrenia.
Another study, of 1,037 people in New Zealand, found those who used cannabis at ages 15 and 18 had higher rates of psychotic symptoms at age 26 than non-users.
A 2011 study in the British Medical Journal of 2,000 teenagers found those who smoked marijuana were twice as likely to develop psychosis as those who didn’t.
Another BMJ study estimated that “13 percent of cases of schizophrenia could be averted if all cannabis use were prevented.”