D-serine, sarcosine, and Ginkgo biloba have been shown to be effective at reducing symptoms of schizophrenia when used as adjunctive treatments to antipsychotic medications.
A meta-analysis of randomized controlled trials found that, when used as an adjunct to antipsychotics, B vitamins, specifically Vitamin B6, Vitamin B12, and Folic Acid, were able to reduce total psychiatric symptoms in people with schizophrenia. However, the results were highly variable, and it was not entirely clear which specific vitamins or amounts determined the effect.[1]