Purely anecdotal, but a long time ago I read a book by Mark Vonnegut, Kurt Vonnegut's son. He explained how he was diagnosed with schizophrenia, and warned that

Purely anecdotal, but a long time ago I read a book by Mark Vonnegut, Kurt Vonnegut's son. He explained how he was diagnosed with schizophrenia, and warned that for him, marijuana made his condition much, much worse, and he learned early on to strictly avoid it. From that one could argue that maybe pot aggravates borderline cases of schizophrenia to the point of a temporary psychotic breakdown & diagnosis. In Mr Vonnegut's case he needed long-term treatment for his schizophrenia either way, but the marijuana effect was short-term and remedied by just staying the heck away from it. Also worth noting that 30+ years ago the stuff people smoked was much weaker than some of the high-end product you can buy today.