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:

  • reduces the amount of glucose the liver releases into the bloodstream
  • improves insulin sensitivity — muscles absorb glucose more effectively
  • may reduce sugar absorption from the gut slightly

Unlike some diabetes medicines, metformin does not stimulate the pancreas to release more insulin — so it rarely causes hypoglycaemia (low blood sugar) when taken alone.

Who takes metformin

Type 2 diabetes — usually first medicine prescribed alongside lifestyle changes (diet, exercise, weight management). Continued long-term to reduce blood sugar and cardiovascular risk.

PCOS — improves insulin resistance, may restore ovulation and regular periods.

Prediabetes — occasionally prescribed for people at high risk of progressing to diabetes.

Metformin is not used for type 1 diabetes.

How to take it

Standard tablets — typically once or twice daily with or after food.

Modified-release (MR/slow-release) — once daily with the evening meal. Swallow whole — do not crush or chew.

Always take with food to minimise stomach upset. Your dose usually starts low and increases gradually.

Common side effects

Most common — affecting up to 1 in 10 people, often improving over weeks:

  • nausea and vomiting
  • diarrhoea or loose stools
  • stomach ache
  • loss of appetite
  • metallic taste in the mouth

Tips to reduce side effects:

  • always take with food
  • ask about modified-release formulation if standard tablets cause diarrhoea
  • start on the lowest dose and increase as directed

Long-term monitoring

Long-term metformin use can lower vitamin B12 levels in some people. A GP may check B12 periodically, especially if you develop anaemia symptoms — tiredness, pins and needles. See our vitamin B12 deficiency guide.

Regular kidney function blood tests ensure metformin remains safe — it is avoided or dose-reduced if kidney function declines significantly.

When to stop metformin temporarily

Stop metformin and contact a GP if you become severely unwell with:

  • vomiting and unable to keep fluids down
  • severe dehydration
  • serious infection

Also paused before surgery and some CT scans with contrast dye — follow hospital instructions. Restart when eating and drinking normally and a doctor confirms it is safe.

Lactic acidosis — rare but serious

Metformin very rarely causes lactic acidosis — a build-up of lactic acid in the blood. Risk increases with severe kidney disease, heart failure, liver disease, or heavy alcohol use.

Warning signs — seek urgent help:

  • muscle cramps or pain
  • severe tiredness or weakness
  • fast or difficult breathing
  • feeling cold, shivering
  • stomach pain with nausea

This is rare with appropriate prescribing and monitoring.

Metformin and lifestyle

Metformin works best alongside:

  • healthy eating — balanced diet, limiting refined carbohydrates
  • regular physical activity
  • weight management if overweight

It is a tool, not a substitute for lifestyle change.

Metformin vs other diabetes medicines

If metformin alone does not control blood sugar, a GP adds other medicines — SGLT2 inhibitors, DPP-4 inhibitors, sulfonylureas, or insulin depending on individual needs. Metformin usually continues as the foundation unless not tolerated.

If you miss a dose

Take it when you remember with food, unless it is nearly time for the next dose — then skip the missed dose. Do not double up.

Common questions

What is metformin used for?
Primarily type 2 diabetes — it lowers blood sugar and reduces cardiovascular risk. Also used off-label in polycystic ovary syndrome (PCOS) to improve insulin resistance, cycles and fertility. Occasionally for prediabetes in high-risk people.
How should I take metformin?
Usually with or after food to reduce stomach side effects. Standard release — once or twice daily with meals. Modified release — once daily with the evening meal. Swallow tablets whole; do not crush modified-release tablets. Follow your prescription exactly.
What are the side effects of metformin?
Common — nausea, diarrhoea, stomach pain, loss of appetite, metallic taste. Usually mild and improve over days to weeks. Starting on a low dose and increasing gradually helps. Rare — vitamin B12 deficiency with long-term use; a GP may monitor levels.
Can metformin cause low blood sugar?
Metformin alone rarely causes hypoglycaemia because it does not stimulate insulin release. Risk increases if combined with sulfonylureas, insulin, or if you skip meals after taking other diabetes medicines alongside it.
What is lactic acidosis with metformin?
A very rare but serious build-up of lactic acid, usually when metformin accumulates during severe illness, kidney failure, or dehydration. Symptoms include muscle pain, extreme tiredness, fast breathing, and feeling cold. Stop metformin when severely unwell and seek urgent help if symptoms occur.

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