One purpose of this study is to test whether adding metformin will limit some of the unwanted effects of clozapine, compared to not adding metformin. Metformin is a medication that is approved by the United States Food and Drug Administration (FDA) for the treatment of type-2 diabetes. Studies have found that people with type-2 diabetes often lose some weight when they take metformin, however the FDA has not approved metformin for weight loss, so for this study the use of metformin is investigational. This study will test whether metformin can help people with schizophrenia or schizoaffective disorders lose weight.