Methods In this 12-week, randomized, double-blind, placebo-controlled trial, metformin 500 mg/d or 1000 mg/d or placebo was prescribed to clozapine-treated patients with schizophrenia having pre-existing metabolic abnormalities. The study was approved by an institutional review board and was conducted at Taipei Medical University-Wan Fang Hospital and Taipei City Psychiatric Center from May 2013 to January 2015. All clinical investigation had been conducted according to the principles expressed in the Declaration of Helsinki. The investigators screened clozapine-treated patients in the first phase and enrolled eligible patients in this clinical trial. Written informed consent was obtained from all patients before the screening.