 Eligible women who gave written informed consent were assigned to 6 months of treatment with either 850 mg of metformin or placebo twice daily, followed by a wash-out period of 3 months before cross-over to the alternate treatment for another 6 months. Randomization defining treatment sequence was done at inclusion by random number tables. The appearance of the tablets was identical, and patients and investigators were blinded to treatment allocation. The randomization code was stored in a closed envelope until all participants had finished the treatment. Participants were seen by one of the investigators before inclusion and every second month during treatment periods, always in the morning after an overnight fast of at least 8 h. They were weighed wearing light clothing. Waist circumference was measured at the umbilical level and hip circumference at the trochanter region. Systolic and diastolic blood pressure was measured with a semiautomatic blood pressure monitor and a blood sample was drawn for immediate analysis without respect to bleeding periods. All participants registered their bleeding periods in a calendar during both study periods and the 3 months wash-out period. Calculation of sample size was based on the assumption that at least 50 of the women would experience at least 30 more menstrual periods on metformin than on placebo. Based on a power of 90 b 0.10 to detect a significant difference two-sided P-value of 0.05, the minimum sampling size was calculated to 44 subjects. 