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Metformin is a medicine used to treat type 2 diabetes.

It’s available on prescription as tablets and as a liquid that you drink.

There are also several different combination tablets for type 2 diabetes. These may contain metformin together with another medicine.

Metformin may also be called by the brand names, Glucophage, Metophage, Siofor, Yaltormin and Lucomet.

Uses of metformin

Metformin lowers your blood sugar levels by improving the way your body handles insulin.

It's usually prescribed for diabetes when diet and exercise alone have not been enough to control your blood sugar levels.

Metformin may also be used to treat polycystic ovarian syndrome (PCOS). It's not approved for this use, but your GP may prescribe it 'off label'. Ask to your GP or pharmacist if you would like more information about off-label use.

Get emergency help

You might need emergency help if you have a serious allergic reaction (anaphylaxis), serious side effects or take too much.

If you take too much

Taking too many metformin tablets can lead to serious health problems.

Emergency action required: Go to an emergency department (ED) immediately if:

  • you take too many metformin tablets

Stop taking metformin. Take the metformin packet or leaflet inside it, and any remaining medicine with you to the ED.

Symptoms of an overdose are severe and quick to appear. They may lead to a coma.

They include:

  • stomach pain
  • difficulty breathing
  • feeling cold
  • unusual sleepiness
  • tiredness or weakness
  • vomiting
  • general feeling of not being well
  • reduced heartbeat

Serious allergic reaction

Be aware of the warning signs of a serious allergic reaction (anaphylaxis). You might need to go to hospital. 

Emergency action required: Call 999 or 112 or go to an emergency department (ED) immediately if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

Serious side effects

Serious side effects are rare.

Urgent advice: Call your GP immediately and stop taking metformin if you get:

  • a general feeling of not being well with severe tiredness, difficulty breathing, being cold and a slow heartbeat
  • vomiting or bellyache with muscle cramps
  • yellow skin or the whites of your eyes turn yellow - these can be signs of liver problems
  • extreme tiredness, lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness and disturbed vision - these could be signs of vitamin B12 deficiency anaemia
  • a skin rash, redness or itching - this could be a sign of a skin disorder
  • a condition affected by dehydration and have severe vomiting, diarrhoea, fever, exposure to heat or you drink less fluid than normal - this could increase the risk of lactic acidosis

When you start taking metformin.

You may not have any symptoms of diabetes, so you may not feel any different when you take metformin.

This does not mean that metformin is not working, so keep taking it.

Take metformin with or straight after a meal to reduce the side effects.

The most common side effects are:

  • feeling and being sick
  • diarrhoea
  • stomach ache
  • not wanting to eat

Metformin does not cause weight gain, unlike some other diabetes medicines.

Eating and drinking

You should cut down on foods with added sugar.

Be careful eating food and drink containing karela (bitter melon). It can lower your blood sugar levels and mean your diabetes is not controlled as well as it should be.

Drinking alcohol

You can drink alcohol while taking metformin. But do not drink more than 2 units per day.

Drinking more than this can increase the risk of low blood sugar and increase the risk of lactic acidosis, a serious side effect.

Check if you can take metformin

Metformin is prescribed for adults, and children aged 10 years and older.

Check with your GP before starting to take metformin if you:

  • already have a medical condition
  • have had an allergic reaction to medicine in the past
  • are pregnant, trying to get pregnant or breastfeeding
  • drink a lot of alcohol
  • have dehydration due to long-lasting or severe diarrhoea, or vomiting

Pregnant and breastfeeding

Tell your GP if you are pregnant, think you might be pregnant or are trying to get pregnant.

You need insulin to treat your diabetes during pregnancy. But your GP may need to change your treatment.

Metformin is not recommended if you are breastfeeding or if you are planning to breastfeed your baby.

How and when to take it

Take metformin tablets with or after a meal to reduce the side effects. Swallow your metformin tablets whole with water. Do not chew them.

Metformin tablets come in different strengths. Your doctor will tell you how many tablets to take a day.

When you first start taking metformin, you'll be advised to increase the dose slowly. This reduces the chances of getting side effects.

If you forget to take it

Do not take a double dose to make up for a forgotten dose.

If you miss a dose of metformin, take the next dose at the usual time. 

Side effects

Stop taking metformin and talk to your GP or pharmacist if side effects bother you or do not go away.

Side effects include:

See the patient information leaflet that comes with your medicine for a full list of side effects.

Non-urgent advice: Find your patient information leaflet

Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.

Report side effects

You can report any suspected side effects to the the Health Products Regulatory Authority (HPRA): report an issue -

Risk of lactic acidosis

Lactic acidosis is a very rare and very serious side effect of taking metformin, particularly if your kidneys are not working properly.

You are at greater risk of developing lactic acidosis if:

  • your diabetes is not controlled
  • you have a serious infection
  • you fast for long periods
  • you drink a lot of alcohol
  • you are dehydrated
  • you have liver problems
  • you have any medical condition that reduces the supply of oxygen to parts of your body, such as acute severe heart disease

Talk to your GP for advice.

Taking metformin with other medicines

Some medicines interfere with the way metformin works.

If you’re taking any other medicines or supplements, check with a GP or pharmacist before you start taking metformin. 

You might need a small adjustment to your metformin dose if you start taking contraceptive pills.

This is because contraceptive pills change how your body handles sugar.

Ask your GP for advice if you need to have:

  • an x-ray or scan involving the injection of contrast medicines
  • surgery

Fact check

This content was fact checked by a pharmacist, a GP, the National Medication Safety Programme (Safermeds) and the Health Products Regulatory Authority (HPRA).

Page last reviewed: 24 September 2021
Next review due: 24 September 2024

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.