Quick answer
What is metformin used for?
Metformin is the first-choice medicine for type 2 diabetes — it lowers blood sugar by reducing glucose production in the liver and improving insulin sensitivity. It also helps some women with PCOS. Common side effects include nausea and diarrhoea, often settling after a few weeks. Take it with food and follow your prescribed dose.
What is metformin?
Metformin is the most widely prescribed medicine for type 2 diabetes in the UK. It lowers blood sugar without causing weight gain — and may help with modest weight loss — by reducing glucose production in the liver and improving the body’s response to insulin. It has been used for decades and has a well-established safety profile when taken correctly.
It is also used in polycystic ovary syndrome (PCOS) to improve insulin resistance and menstrual regularity.
How metformin works
Metformin lowers blood sugar in several ways: it reduces the amount of glucose the liver releases into the bloodstream, it improves insulin sensitivity so muscles absorb glucose more effectively, and it may also slightly reduce sugar absorption from the gut.
Unlike some diabetes medicines, metformin does not stimulate the pancreas to release more insulin, which is why it rarely causes hypoglycaemia (low blood sugar) when taken alone.
Who takes metformin
For type 2 diabetes, metformin is usually the first medicine prescribed alongside lifestyle changes such as diet, exercise and weight management, and it is often continued long-term to keep blood sugar down and reduce cardiovascular risk. For PCOS, it improves insulin resistance and may help restore ovulation and regular periods. It is occasionally prescribed for prediabetes in people at high risk of progressing to diabetes. Metformin is not used for type 1 diabetes.
How to take metformin
Standard tablets are typically taken once or twice daily with or after food. Modified-release (MR, or slow-release) tablets are taken once daily with the evening meal and should be swallowed whole rather than crushed or chewed. Taking metformin with food helps minimise stomach upset, and your dose usually starts low and increases gradually.
Common side effects of metformin
The most common side effects affect up to 1 in 10 people and often improve over a few weeks. They include nausea and vomiting, diarrhoea or loose stools, stomach ache, loss of appetite, and a metallic taste in the mouth.
A few simple steps can help reduce these side effects: always take metformin with food, ask about the modified-release formulation if standard tablets cause diarrhoea, and start on the lowest dose before increasing as directed.
Long-term monitoring on metformin
Long-term metformin use can lower vitamin B12 levels in some people, so a GP may check B12 periodically, especially if you develop symptoms of anaemia such as tiredness or pins and needles (see our vitamin B12 deficiency guide). Regular kidney function blood tests also help ensure metformin remains safe, since it is avoided or dose-reduced if kidney function declines significantly.
When to stop metformin temporarily
Stop taking metformin and contact a GP if you become severely unwell with vomiting and cannot keep fluids down, with severe dehydration, or with a serious infection. It is also usually paused before surgery and before some CT scans that use contrast dye, so follow hospital instructions closely. Restart only once you are eating and drinking normally and a doctor has confirmed it is safe to do so.
Lactic acidosis — a rare but serious risk
Metformin very rarely causes lactic acidosis, a build-up of lactic acid in the blood. The risk increases with severe kidney disease, heart failure, liver disease, or heavy alcohol use.
Seek urgent help if you notice any of these warning signs: muscle cramps or pain, severe tiredness or weakness, fast or difficult breathing, feeling unusually cold or shivering, or stomach pain together with nausea. Lactic acidosis is rare when metformin is prescribed and monitored appropriately.
Metformin and lifestyle
Metformin works best alongside healthy eating with a balanced diet that limits refined carbohydrates, regular physical activity, and weight management if you are overweight. It is a tool that supports these changes rather than a substitute for them.
Metformin compared with other diabetes medicines
If metformin alone does not control blood sugar, a GP may add other medicines such as SGLT2 inhibitors, DPP-4 inhibitors, sulfonylureas or insulin, depending on individual needs. Metformin usually continues as the foundation of treatment unless it is not tolerated.
What to do if you miss a dose
If you miss a dose of metformin, take it when you remember with food, unless it is nearly time for your next dose, in which case skip the missed one. Never take a double dose to make up for a missed one.
Common questions about metformin
- What is metformin used for?
- Metformin is used mainly for type 2 diabetes, where it lowers blood sugar and helps reduce cardiovascular risk. It is also used off-label for polycystic ovary syndrome (PCOS) to improve insulin resistance, cycle regularity and fertility, and is occasionally prescribed for prediabetes in people at high risk.
- How should I take metformin?
- Metformin is usually taken with or after food to reduce stomach side effects. Standard-release tablets are taken once or twice daily with meals, while modified-release tablets are taken once daily with the evening meal and swallowed whole rather than crushed. Always follow your prescription exactly.
- What are the side effects of metformin?
- The most common side effects are nausea, diarrhoea, stomach pain, loss of appetite and a metallic taste, and these are usually mild and improve over days to weeks. Starting on a low dose and increasing gradually can help. With long-term use, vitamin B12 deficiency is a rarer possibility that a GP may monitor for.
- Can metformin cause low blood sugar?
- Metformin taken alone rarely causes hypoglycaemia because it does not stimulate insulin release. The risk increases if it is combined with sulfonylureas or insulin, or if meals are skipped while taking other diabetes medicines alongside it.
- What is lactic acidosis with metformin?
- Lactic acidosis is a very rare but serious build-up of lactic acid, usually occurring when metformin accumulates in the body during severe illness, kidney failure or dehydration. Symptoms include muscle pain, extreme tiredness, fast breathing and feeling cold. Stop taking metformin if you become severely unwell and seek urgent help if these symptoms occur.