I'm speaking as a medical student. And I can't speak for what it's like in the US - only Sweden. Here it's not uncommon for schizofrenic patients to self medicate with marijuana. I guess it increases their quality of life until they have just a little too much and end up having a psychosis. That said, at least among Swedish psychiatrists, it's well know that marijuana can trigger schizophrenia in people that are ~16 years old.