My former B-I-L (ex hubby's brother) was a major league pot smoker who quickly developed severe paranoid schizophrenia, severe enough that he was Baker Act'ed into custody twice for being a danger to himself and others. Eventually, he killed himself with a shotgun when the voices got too loud.
I personally follow the theory that he already had a mental illness, and the pot (plus the occasional snort of cocaine) just made it worse. Lots of mental and physical conditions can be exacerbated by substance abuse - alcohol especially.
I do not believe pot causes schizophrenia - I think that is the anti-pot lobby trying to create another cause-effect scenario to justify continued prohibition. An example- people who have previously unknown congenital heart defects can die the first time they use cocaine. (Len Bias) That does not mean that cocaine is more likely to cause a heart condition but cocaine can certainly fuck you up if your heart can't handle the stress.