Polycystic ovary syndrome (PCOS) is a common endocrine disorder with diverse reproductive and metabolic features. It is underpinned by insulin resistance that is exacerbated by obesity. Lifestyle modification is the first line treatment in PCOS, but it is associated with low adherence and sustainability. In small studies, metformin improves outcomes such as hyperinsulinaemia, ovulation and menstrual cyclicity. We conducted a systematic review and meta-analysis to compare the effect of lifestyle modification + metformin with lifestyle modification ± placebo, and of metformin alone with lifestyle modification ± placebo in PCOS on anthropometric, metabolic, reproductive and psychological outcomes.