AIM: Some of our obese patients who were receiving 10 mg/day sibutramine reported feeling hunger at night. To address this, we designed a randomized, prospective clinical trial to study the efficacy and safety of 10 mg sibutramine twice daily (bid), and compare this treatment with 120 mg orlistat three times daily (tid) and 850 mg metformin (bid). METHODS: A total of 150 female patients with body mass index (b.m.i.) > 30 kg/m(2) were included. The subjects were all out-patients at the Başkent University Endocrinology and Metabolism Clinic. Each individual was assigned randomly to receive 10 mg sibutramine bid (group 1; n = 50; mean age 42.27 +/- 1.40 years), 120 mg orlistat tid (group 2; n = 50; mean age 42.13 +/- 1.32 years) or 850 mg metformin bid (group 3; n = 50; mean age 43.58 +/- 1.40 years). All patients took the medications for 6 months. Two patients from the sibutramine group and two from the orlistat group were withdrawn from the study because of side-effects. RESULTS: After 6 months of treatment, the sibutramine, orlistat, and metformin groups all showed significantly reduced b.m.i. (13.57%, 9.06% and 9.90% respectively); waist circumference (10.43%, 6.64%, and 8.10% respectively); fasting and postprandial blood glucose levels; insulin resistance as assessed by the homeostasis model for assessment of insulin resistance (HOMA) (38.63%, 32.73% and 39.28%, respectively); levels of total cholesterol, low-density lipoprotein (LDLC) cholesterol, very low-density lipoprotein (VLDLC) cholesterol, triglyceride, lipoprotein (a), and apolipoprotein B; uric acid level; pulse rate; and systolic and diastolic blood pressure. None of the groups showed any significant changes in levels of high-density lipoprotein (HDLC) cholesterol, or apolipoprotein A1. There was a significantly greater fall in b.m.i. in the sibutramine group than in either of the other groups (p < 0.0001). CONCLUSIONS: The results of this study confirm that sibutramine, orlistat and metformin are all effective and safe medications that reduce cardiovascular risk and can decrease the risk of type 2 diabetes mellitus in obese females. Overall, treatment with 10 mg sibutramine bid is more effective than orlistat or metformin therapy in terms of weight reduction.