While niacinamide has been used as an adjunct supplement to anti-psychotic medications in the hopes that it might help to preserve cell function, decrease anxiety, and lessen the symptoms of schizophrenia, the evidence that it does these things is highly anecdotal. In fact, the original studies done in the 60s and 70s that touted niacinamide as a wonder-cure have yet to be replicated.
The fact is that niacinamide can have many side effects and interaction with other drugs if not overseen by a physician. Also, in high doses (several hundred milligrams to a few grams) permanent damage can be done to the liver as well as the eyes. Common side effects include nausea, other gastrointestinal distress and flushing of the skin.
Thus, if using niacinamide as a supplement you should check with the treating physician regarding dosage and interactions. Never take very high doses (some websites have patients claiming to take up to several grams a day!) without first consulting your doctor. The maximum recommended dosage (via the Mayo Clinic) is 35mg a day, taken with meals.
I hope this helps - please let me know if you have more questions about it.