Well, you can't call it anything other than correlative (i.e. people who smoke weed are more likely to have schizophrenia than the general population). You can see that and say "Marijuana causes/ is a factor in the development of Schizophrenia", but it's equally viable to say "Schizophrenics often self-medicate with marijuana".
Don't get me wrong, I come down on the side of "Marijuana is probably not the best thing to be using to ease the effects of mental illness, unless encouraged and observed by a medical professional", but I don't like coarse inferences.

OP: Consult a medical practitioner.