Metformin is a drug used to treat diabetes but one of its off-label uses is to treat Polycystic Ovarian Syndrome. What does metformin do for someone who has PCOS but does not have diabetes? Is metformin necessary to treat PCOS?
PCOS can make your body resistant to insulin. When your body's cells become too resistant to insulin, that's called type 2 diabetes. Metformin makes your body more sensitive to insulin, so it can help prevent type 2 diabetes in PCOS patients who do not yet have it.
Metformin doesn't treat PCOS itself, and it's not approved by the FDA to treat PCOS. Its use is appropriate when the patient shows early signs of insulin resistance, a condition called prediabetes.
This page has some good info: http://youngwomenshealth.org/2014/02/25/metformin/.
Incidentally, some physicians & scientists believe that metformin can be used to treat other symptoms of PCOS. If you're interested in a critical review of those claims, see this paper. Here's a key paragraph from the conclusion:
The use of metformin in PCOS has received a lot of attention for obvious reasons. Once thought of as a wonder drug, the accumulating evidence on the efficacy of metformin has been disappointing. The lack of an emphatic or overwhelming efficacy is largely due to the patients' variability in phenotypes and their metabolic parameters. Some studies have tried to identify the patients that are most likely to benefit from metformin, yet again the results have not been forthcoming. Consequently the burden falls back on the clinician who should be familiar with the gist of the available evidence to be able to identify the right patient for the treatment in hand. Obtaining an evidence of IR [insulin resistance] is a good starting point prior to recommending its use.
Please keep in mind PCOS is a very heterogenous disorder, meaning that any two women can have PCOS with totally different clinical manifestations and also metabolic consequences. Not everyone will need metformin, because not everyone has the same metabolic consequences of PCOS.