Does metformin break a fast?

A metformin tablet provides no energy, sugar or protein. So the tablet itself does not break a fast for any of this site's four goals. Metformin lowers blood sugar. It treats type 2 diabetes and diabetes in pregnancy, and the symptoms of PCOS (now also called PMOS).

The bigger question is food. Most people take metformin 1 to 3 times a day with food, and the US label says to take it with meals. Common side effects include feeling sick, being sick, diarrhoea, tummy pain and loss of appetite. Over time it can also lower vitamin B12, which can make you feel very tired. Limit alcohol while you take metformin, as it can increase the risk of serious side effects.

Take metformin exactly as prescribed, with food if your prescription says so. If your dose times fall inside the fasting window you planned, ask your doctor or pharmacist how to fit the fast around the medicine. Never move, skip or stop a dose to protect a fast.

Fasting changes blood sugar, and diabetes medicines may need adjusting by your diabetes team. Side effects may be more likely if you take more than one blood-sugar-lowering medicine, such as metformin with insulin. If you take insulin or a sulfonylurea, do not fast without medical advice. Break any fast if blood glucose falls below 3.9 mmol/L (70 mg/dL) or rises above 16.7 mmol/L (300 mg/dL).

Vomiting, diarrhoea or a fever while taking metformin puts you at risk of dehydration. If that happens, stop fasting and talk to a pharmacist or doctor.