BiguanidePrescription only

Metformin

Tablet (standard or slow-release)

Quick Summary

Metformin lowers blood sugar by reducing the amount of sugar the liver releases and helping the body respond better to insulin. It doesn't usually cause low blood sugar on its own and doesn't cause weight gain.

Type 2 diabetes

Gestational diabetes

Polycystic ovary syndrome (PCOS)

Medical Uses

Type 2 diabetes: The usual first medicine, alongside healthy eating and activity.

Gestational diabetes: Sometimes used during pregnancy when diet alone isn't enough.

Polycystic ovary syndrome (PCOS): Sometimes used, on a doctor's advice, to help with insulin resistance.

How it's taken

Metformin is taken with or just after meals to reduce stomach upset, once or several times a day depending on the type of tablet. Your doctor decides the right dose for you and may change it over time.

Doctors often start with a small amount and increase it slowly to help your stomach get used to it. Slow-release tablets should be swallowed whole.

Your doctor decides the right dose for you. Follow their instructions and the label.

Common Side Effects

Stomach upset

Feeling sick, diarrhoea, tummy pain or loss of appetite, especially at first. Taking it with food helps.

Metallic taste

Some people notice a metallic taste in the mouth.

Low vitamin B12

Long-term use can lower vitamin B12. Your doctor may check it.

Interactions

Alcohol

Heavy drinking increases the risk of a rare but serious side effect (lactic acidosis).

Contrast dye for scans

Metformin may need to be paused around some X-rays or scans that use dye. Tell the radiology team you take it.

Other diabetes medicines

Combined with insulin or some tablets, it can contribute to low blood sugar.

Critical Warnings

Lactic acidosis

Black Box Warning: Rare but serious. Get urgent help if you feel very unwell with vomiting, tummy pain, fast breathing, muscle pain or unusual sleepiness.

Kidney problems

Your doctor checks your kidney function, as metformin may need to be changed or stopped if the kidneys aren't working well.

Being unwell or dehydrated

If you're vomiting, have diarrhoea or can't drink fluids, ask your doctor whether to pause metformin.

Sources and further reading

Last updated 21 October 2026. For more detail, see these trusted health organisations.

Metformin(opens in a new tab) · NHS
Metformin: drug information(opens in a new tab) · MedlinePlus, US National Library of Medicine