There IS conclusive and substantial evidence that cannabis increases your risk for psychosis (beyond the transient psychosis seen during intoxication). It's perhaps the only major negative effect that's been found so far. I wrote a whole post on this and other possible effects cannabis may have but for some reason it got deleted.
Someone above is also correct that genetic factors may influence the risk of psychosis in adults who used cannabis as adolescents which may explain why the vast majority of users do not develop symptoms. 1 and 2
Now I'm not saying that a person's risk of becoming schizophrenic is high, but it IS increased. Our lecturer on marijuana and drug addiction did emphasize that based on the evidence and current literature out there, if an adolescent or anyone with any family history of any mental illness, including depression, smokes marijuana, we especially should warn them of the risks and try to help them stop.