Twenty years of ovulation induction with metformin for PCOS; what is the best available evidence?

The potential reproductive benefits of metformin, a drug endowed with the capacity to ameliorate insulin resistance in polycystic ovary syndrome (PCOS), has garnered much interest over the past 2 decades. In this review, randomized-controlled trials (RCT) and meta-analyses of RCT comparing metformin are critically appraised and summarized. PubMed and CENTRAL databases were consulted. Evidence is insufficient to favour the use of metformin or metformin plus clomiphene citrate instead of clomiphene citrate for ovulation induction in women with newly diagnosed PCOS. Evidence is also insufficient to recommend metformin as a primary treatment for non-obese women with PCOS. Metformin plus clomiphene citrate should be considered as an effective option in clomiphene citrate-resistant PCOS. In women with PCOS undergoing gonadotrophin ovulation induction, metformin significantly increased pregnancy and live birth rates (P < 0.0001 and P = 0.020, respectively) with reduced risk of cancelled cycles. A beneficial effect of metformin co-treatment in increasing clinical pregnancy rates and reducing the risk of OHSS in PCOS patients undergoing assisted reproduction techniques has been shown. No evidence was found of reduced risk of spontaneous abortion or increased risk of major anomalies in women with PCOS taking metformin during the first trimester.