Efficacy of Pimozide Augmentation for Clozapine Partial Response

This is a 12 week outpatient study for patients with schizophrenia who are on Clozapine, but continue to experience symptoms. The purpose of this project is to find out if small doses of pimozide (an antipsychotic medication, taken by mouth) will be helpful in reducing symptoms (such as hearing voices, having trouble in organizing your thoughts, lack of interest in life events and social activities), compared to placebo (an inactive substance, \"sugar pill\"), when added to clozapine in patients with schizophrenia.

The participant will be asked to come in once a week to meet with the research staff and study doctor. The participant will continue to see your regular clinician during this study for all normal appointments. The participant will remain on your current medications throughout the study. During the study you will be randomly selected to be put on a small dose of Pimozide or placebo.