PCOS occurs when a woman does not release an egg regularly each month, causing her periods to be irregular. Women with PCOS can also have increased hair growth on the face and body, acne, head balding, infertility, pre-diabetes, and diabetes. PCOS is commonly treated with oral contraceptive pills (also known as the birth control pills). Sometimes, a medication called metformin is also used to treat PCOS, especially if a woman has evidence of insulin resistance or if fertility is desired. Unfortunately, metformin works in only some women with PCOS. The mechanism through which metformin works in PCOS is not clear and it difficult to predict who will benefit from metformin treatment and who will not.