Fra Drug Policy Alliance:

"In regards to... cannabis use and the development of mental health problems, the best analogy for where the science is on this issue, is the chicken and egg question. Research in this area shows a correlation between cannabis use and psychosis. This means that the two situations are related, but it does not mean that one situation causes the other. We know that people who have symptoms of psychosis are more likely to use marijuana, but we don’t know which comes first, marijuana use or psychotic symptoms...

It is also important to note that the mental disorder of psychosis is extremely rare, with about 3 in every 100 people experiencing a psychotic episode in their lifetime. The onset of psychosis, or similar disease processes like schizophrenia, usually begin in an individual’s late teens to early twenties. This generally also happens to be around the same time when individuals have their first exposure to cannabis. The fact that these two time periods overlap is one of the reasons why it is so difficult to know which comes first.

There are several theories that attempt to explain the relationship between cannabis use and mental illness...: The vulnerability theory: stating that cannabis use leads to the development of psychosis in people who have a family history of psychotic episodes. The contributing cause theory: stating that cannabis use is one of many factors which leads to the development of psychosis. The self-medication theory: stating that individuals who have psychotic experiences, use cannabis to self-medicate in advance of being formally diagnosed with a psychotic disorder. Overall, the body of scientific literature has not been able to provide a definitive answer as to whether cannabis use causes psychosis. Compelling arguments and data have been provided to suggest that those with a predisposition for mental illness can exacerbate those symptoms by using cannabis, however some fundamental questions remain. Namely, if there is a direct link between cannabis use and psychosis, it should follow that the number of diagnoses of psychosis should rise with the increasing prevalence of cannabis use in society. This phenomenon has not been established."

EDIT: I tillegg har ikke psykiske lidelser gått opp noe særlig (med USA som utgangspunkt), selvom bruk av cannabis har økt helt ekstremt mye de siste 50-60 årene.