There are questions of whether a link exists between cannabis use and adult-onset schizophrenia (ie, a "psychotic break" in the early 20s).

The problem is that these variables are highly confounded - not only is it reasonable, but it is almost expected that a person who is experiencing early signs of schizophrenia (even if they don't realize it) might have their life impacted enough that they turn to alcohol or marijuana (or other drugs) to address those secondary problems, even though schizophrenia would be the underlying root "cause".

The question is not whether that second effect (schizophrenia causing drug use) exists, but the magnitude of that relationship. Which is difficult to measure on a theoretical level, and perhaps impossible to measure on an ethical level.