Polycystic ovary syndrome (PCOS) is one of the most frequent diseases that affects women in their reproductive age. The heterogeneity of PCOS makes not only the diagnosis but also the choice of an adequate treatment difficult. The biguanide, N, N' dimethyl-biguanide: Metformin is an antidiabetic drug that increases glucose utilization in insulin-sensitive tissues and is useful in the reduction of both insulin resistance and circulating androgens as well as restoring ovulation. However, metformin is being clinically used without a complete understanding of the mechanism involved. The present review explores some of the actions and efficacy of metformin in the treatment of PCOS during different reproductive periods.