Context: Polycystic ovary syndrome (PCOS) is common among women of childbearing age and the available pharmacological therapies have different side-effect profiles. Objective: We summarized the evidence about the side effects of oral contraceptive pills, metformin, and anti-androgens in women with PCOS. Data Source: Sources included Ovid Medline, OVID EMBASE, OVID Cochrane Library, Web of Science, Scopus, PsycInfo, and CINAHL from inception through April 2011. Study Selection: We included comparative observational studies enrolling women with PCOS who received the agents of choice for at least 6 months and reported adverse effects. Data Extraction: Using a standardized, piloted, and Web-based data extraction form and working in duplicate, we abstracted data from each study and performed meta-analysis when possible. Data Synthesis: We found 22 eligible studies of which 20 were randomized. No study reported severe side effects (eg, lactic acidosis, thromboembolic episodes, liver toxicity, cancer incidence, or pregnancy loss). Meta-analysis demonstrated no significant change in weight in oral contraceptive pills or flutamideusers. Indirect evidencefrompopulations withoutPCOSdemonstratednoincreased risk of lactic acidosis with metformin, only case reports of liver toxicity with flutamide (no comparative evidence), andincreased relative risk differenceofvenousthromboembolismwithoral contraceptive pillsbutvery low absolute risk. Evidence on mortality, cardiovascular mortality, and cancer was inconclusive. Conclusions: Drugs commonly used to treat PCOS appear to be associated with very low risk of severe adverse effects although data are extrapolated from other populations. © 2013 by The Endocrine Society.