This study from October 2021 examined the hypothesis that adolescent cannabis use was a cause of psychosis. This is a frequently mentioned possible adverse effect of pre-adult cannabis use. It was published in the Journal of Abnormal Psychology.
They found that diagnosed adult psychosis was "likely attributable to familial confounds rather than a causal effect of cannabis exposure." That is pretty clear, but how did they come to this conclusion?
The author's analysis combines data from two longitudinal studies at the Minnesota Center for Twin and Family Research (MCTFR). They assessed 2 cohorts of twins for psychosis-proneness using the Psychoticism scale of the Personality Inventory for DSM–5, administered in adulthood. The MCTFR study was designed specifically to examine the effects of adolescent substance use on later mental health, so the data is pretty focused.
Participants came from two longitudinal studies of same-sex twin pairs at the MCTFR. Cohort 1 had 998 people and Cohort 2 had 1512. They were assessed every 3-7 years. Over 30% of the participants reported at least some cannabis use in adolescence.
One in seven met criteria for adolescent cannabis use disorder, and close to one in ten reporting at least one past-year period of at least weekly cannabis use.
The results in a nutshell:
This study, however, does not support these hypotheses, suggesting instead that observed associations are more likely due to confounding by common vulnerability factors.
Although cannabis use and disorder are consistently associated with increased risk of psychosis, the present results suggest this association is likely attributable to familial confounds rather than a causal effect of cannabis exposure.
The full text article is here at the US National Library of Medicine website.