Is it the cause of the psychosis, or making it surface earlier than it would have naturally?
30 years ago, I was partying with some friends when an older sibling came to us in tears begging us to not smoke pot with his younger sibling any longer. Their sibling was in their late teens/early 20s and started showing early signs of schizophrenia, that I did not personally see until later and it was clear that things were not ok. It's always stayed in my mind.
My own older sibling was a hardcore alcohol addict before they died, and I suspect they started using because of their uncontrolled bipolar disorder that started surfacing in their mid 20s.