Naltrexone for Antipsychotic-Induced Weight Gain

Inclusion Criteria: * Age 18 to 75 * Meet Diagnostic \& Statistical Manual - 4 (DSM-IV) criteria for schizophrenia, schizoaffective disorder, bipolar disorder, major depression, or another psychotic disorder based on Structured Clinical Interview for the DSM-IV (SCID) interview * Body Mass Index (BMI) of 28 and over * On a stable dose of antipsychotic medication; i.e. at least one month with no dose change, and three months from an antipsychotic switch * Deemed to be symptomatically stable by the clinical staff in the last two months * Over 7% total body weight increase on antipsychotics for subjects within first year of illness Exclusion Criteria: * Meet criteria for current opiate abuse or dependence (confirmed by positive urine drug screen for opiates or, if suspected by study doctor via patient history and or suspicion of occult opiate use, a naloxone challenge will be performed.) * Current history of dementia, mental retardation * Not capable of giving informed consent for participation in the study * Women who are pregnant or breast-feeding * Physical conditions affecting body weight (e.g. Cushing's disease, polycystic ovary syndrome) Diabetes Mellitus (defined as prescribed an anti-diabetic medication for diabetes or a hemoglobin A1c level \> 7 confirmed by primary care physician at screening) * Severe liver dysfunction, (serum aminotransferases greater than three times normal), acute infectious hepatitis, liver failure.