Metformin has been used for alleviating metabolic abnormalities in patients with schizophrenia. Until now, the lowest dose of metformin to treat metabolic abnormalities in clozapine-treated patients is 1000 mg/d. The aim of this study was to determine whether a lower dosage of metformin, such as 500 mg/d, is effective for improving metabolic profiles in clozapine-treated patients with pre-existing metabolic abnormalities.