The studies showing Niacin did anything for schizophrenia are from the 50s, and there hasn’t been anything new in that front.
That said, there is some science to Niacin reducing symptoms, since it increases the amount of kynurenine 3-monooxygenase produced, which tilts tryptophan metabolism away from kynurenic acid formation. Kynurenic acid is suspected to be one of the main causes of schizophrenic symptoms, as an NMDA antagonist.
I haven’t benefitted from Niacin supplementation nearly as much as from Amyloban 3399 supplementation though. If you’ve talked to your psychiatrist and exhausted your antipsychotic options, maybe that is worth a try.
Me and a few other people on this forum have had a huge reduction in positive symptoms from it. There’s a thread about it here: