At this point in the research, psychedelic hallucinogens may not be entirely contraindicated with schizophrenia in a few circumstances.
As for weed, things get more complicated. There are so many substances in weed so that two given strains can feel very different, even if they have the same delta-9-THC levels. Also, phytocannabinoids and the endocannabinoid system have not been well-researched, probably partly by refusal to allow people to use it (S-I status), but also because it's just much more complicated than looking at monoamine, or GABA, or endomorphin activity.
I think a good substantiated example would be that there can be very therapeutic effects and very detrimental effects at the same time, when using the product. It can be easy to kind of push out the view of the negative effects by citing the positive ones. Another idea would be that marijuana can help with a variety of mental illnesses like PTSD, depression, and insomnia, but is possibly as far off as can be when looking at helping psychotic symptoms. Something that can be used to help sleep isn't something that would logically fit in place with regard to also increasing psychotic tendencies.
Another idea would be that people are discriminated against if they have schizophrenia, so that weed could allay that trauma, but again, can make organic symptoms worse. Weed may also help some autistic symptoms in concert with a healthy CBD dose, which can appear similar to some symptoms of schizophrenia, but not the same. People can rationalize use that way, however.
I've known lots of people with schizophrenia who say it helps them. The truth is, it probably helps them and hurts them. But the hurt is of quite greater magnitude than the help, especially with most weed of today. That's what psychiatry agrees upon. Every weed user with schizophrenia whom I know, said it helped them when they were on it, then said it didn't help them at all when they were able to be clean. Also by observation, these people looked considerably more sick when they were smoking weed.
Edit:
Lol vote me for prime minister.
I don't think that doctors do a great job of detailing out one's prognosis. But then again it can't be easy to tell where someone's mind is, especially if that someone is supposed to be exceptional in not thinking like the masses. At that point it's like an "when you grow up, you'll understand" thing, which can feel positively condescending, and made worse because of how harsh we can be to people, including ourselves. I think that most mental health patients have some mood instability already. Pair that with feeling condescended to, habit, social issues, and usually some kind of drug use, and you get a volatile mixture such that most people won't want to trust mental health authorities.
