Polycystic ovary syndrome: metformin in women not planning pregnancy

How well does metformin work?

Seven small studies have looked at how well metformin, used on its own, worked compared with co-cyprindiol. Overall, these studies could not reliably say if metformin was better or worse than co-cyprindiol in improving excessive hair growth and acne. Metformin did not work as well as co-cyprindiol at regulating periods in 2 studies. Four other studies could not reliably say if metformin was better or worse than co-cyprindiol in regulating periods.

Two small studies looked at the combination of metformin and co-cyprindiol compared with co-cyprindiol or metformin used on their own. These studies could not reliably say if there was a difference between these treatments in improving excessive hair growth or regulating periods.

All these studies had only small numbers of women and so these findings may not be accurate. Two other studies looked at metformin compared with oral contraceptives but they did not have enough women to draw reliable conclusions.

The length of treatment in the studies was up to 6 months, so it isn't known how well metformin may work over longer periods. It is also not possible to say from these studies if metformin has any effect in the longer term on the development of diabetes or endometrial cancer.

What are the possible harms or side effects?

Metformin is most often used in people with diabetes. Based on that experience, the most common side effects are problems affecting the stomach and bowel, such as feeling sick (nausea), being sick (vomiting), diarrhoea, stomach (abdominal) pain and loss of appetite. These side effects occur in more than 1 in 10 people taking metformin but they most often happen at the beginning of treatment. Spreading the doses over the day and taking the tablets with or straight after a meal can help reduce these side effects.

Based on the evidence available, more women with PCOS taking metformin had side effects affecting the stomach and bowel than women taking co-cyprindiol. However, co-cyprindiol was more likely than metformin to cause other side effects such as weight gain, high blood pressure, depression, chest pain and headache. The studies comparing metformin and co-cyprindiol had only small numbers of people so these findings may not be accurate.