OBJECTIVES: Diabetes has become one of the major chronic diseases of the 21st century, and has ranked fifth on the main causes of death in Taiwan for years. The high cost of antidiabetic medications expenditure has brought the concern on the generic and brand drugs substitution. The purpose of the study was to investigate the effects of hospital drug brands changes on physicians’ prescription behavior and patients’ outcomes. METHODS: The study was conducted at a medical center in Taipei city. We recruited those who had taken the same medicine regularly from January 2004 to June 2004. In the next 4 years, the study hospital had changed the manufactures of metformin four times. Physicians’ prescription data and patients’ glycated hemoglobin values were collected. We retrospectively analyzed the prescription patterns for oral antidiabetes medications of patients with persistence medication service in 5 different metformin brand stages. RESULTS: The results show 1) the average item of anti-diabetes drugs in each prescription was significantly increased; 2) The prescription rates were significant increased for acarbose and insulin, and decreased for metformin, glibenclamide and rosiglitazone; 3) The daily dose of metformin, which was on average between 1,565 and 1,611 mg, wasn’t increased significantly regardless of the change in manufacturers; and 4) The HbA1c values of study patients were increasing, but no significant difference was observed in different metformin brand stages. CONCLUSIONS: The use of generic drugs can effectively reduce the cost of medication. The results provide information for hospital administrators in drug purchasing and price negotiating with the manufacturers.