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  1. About metformin
  2. Key facts
  3. Who can and cannot take metformin
  4. How and when to take it
  5. Side effects
  6. How to cope with side effects
  7. Pregnancy and breastfeeding
  8. Cautions with other medicines
  9. Common questions

1. About metformin

Metformin is a medicine used to treat type 2 diabetes, and to help prevent type 2 diabetes if you're at high risk of developing it.

Metformin is used when treating polycystic ovary syndrome (PCOS), although it's not officially approved for PCOS.

Type 2 diabetes is an illness where the body does not make enough insulin, or the insulin that it makes does not work properly. This can cause high blood sugar levels (hyperglycaemia).

PCOS is a condition that affects how the ovaries work.

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.

For women with PCOS, metformin lowers insulin and blood sugar levels, and can also stimulate ovulation.

Metformin is available on prescription as tablets and as a liquid that you drink.

2. Key facts

  • Metformin works by reducing the amount of sugar your liver releases into your blood. It also makes your body respond better to insulin. Insulin is the hormone that controls the level of sugar in your blood.
  • It's best to take metformin with a meal to reduce the side effects.
  • The most common side effects are feeling and being sick, diarrhoea, stomach ache and going off your food.
  • Metformin does not cause weight gain, unlike some other diabetes medicines.
  • Metformin may also be called by the brand names Bolamyn, Diagemet, Glucient, Glucophage and Metabet. Liquid metformin is called by the brand name Riomet.

3. Who can and cannot take metformin

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

Metformin is not suitable for some people. Tell your doctor before starting the medicine if you:

  • have had an allergic reaction to metformin or other medicines in the past
  • have uncontrolled diabetes
  • have liver or kidney problems
  • have a severe infection
  • are being treated for heart failure or have recently had a heart attack
  • have severe problems with your circulation or breathing difficulties
  • drink a lot of alcohol

You may need to stop taking metformin before having surgery and certain medical tests. Tell your doctor if you need to have:

  • a test such as an X-ray or scan involving the injection of a dye that contains iodine into your blood
  • surgery where you'll be put to sleep

4. How and when to take it

It's best to take metformin tablets with a meal to reduce the side effects. Swallow your metformin tablets whole with a glass of water. Do not chew them.


The maximum daily dose is 2,000mg a day (for example, 4 x 500mg tablets).

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

Different types of metformin

Metformin comes as 2 different types of tablet: standard-release tablets and slow-release tablets.

Standard-release tablets release metformin into your body quickly. You may need to take them several times a day depending on your dose.

Slow-release tablets dissolve slowly so you do not have to take them as often. One dose is usually enough, and you'll take it with your evening meal.

Your doctor or pharmacist will explain what type of metformin tablets you're on and how to take them.

Metformin is also available as a liquid for children and people who find it difficult to swallow tablets.

Will my dose go up or down?

Your doctor will check your blood sugar levels regularly and may change your dose of metformin if necessary.

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

For example:

  • one 500mg tablet with or after breakfast for at least 1 week, then
  • one 500mg tablet with or after breakfast and your evening meal for at least 1 week, then
  • one 500mg tablet with or after breakfast, lunch and your evening meal

If you find you cannot tolerate the side effects of standard-release metformin, your doctor may suggest switching to slow-release tablets.

What if I forget to take it?

If you miss a dose of metformin, take the next dose at the usual time. Do not take a double dose to make up for a forgotten dose.

If you often forget doses, it may help to set an alarm to remind you.

You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

An overdose of a large number of metformin tablets can cause serious health problems. The symptoms are severe and quick to appear.

They include:

  • stomach pain
  • diarrhoea
  • fast or shallow breathing
  • feeling cold
  • unusual sleepiness
  • tiredness or weakness

Urgent advice: Go to A&E now if:

  • you take too many metformin tablets

Take the metformin packet or leaflet inside it, plus any remaining medicine, with you.

Find your nearest A&E

5. Side effects

Like all medicines, metformin can cause side effects, although not everyone gets them.

Common side effects

Common side effects happen in more than 1 in 100 people.

Talk to your doctor or pharmacist if these side effects bother you or do not go away after 1 week:

  • feeling sick (nausea)
  • being sick (vomiting) or diarrhoea
  • stomach ache
  • loss of appetite
  • a metallic taste in the mouth

Serious side effects

Serious side effects are rare and happen in less than 1 in 10,000 people.

Call your doctor straight away if you get warning signs of:

  • a general feeling of discomfort with severe tiredness, fast or shallow breathing, being cold and a slow heartbeat
  • 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

Low blood sugar

Metformin does not usually cause low blood sugar (known as hypoglycaemia, or "hypos") when taken on its own.

But hypos can happen when you take metformin with other diabetes medicines, such as insulin or gliclazide.

Early warning signs of low blood sugar include:

  • feeling hungry
  • trembling or shaking
  • sweating
  • confusion
  • difficulty concentrating

It's also possible for your blood sugar to go too low while you're asleep.

If this happens, it can make you feel sweaty, tired and confused when you wake up.

Low blood sugar may happen if you:

  • take too much of some types of diabetes medicines
  • eat meals irregularly or skip meals
  • are fasting
  • do not eat a healthy diet and are not getting enough nutrients
  • change what you eat
  • increase your physical activity without eating more to compensate
  • drink alcohol, especially after skipping a meal
  • take some other medicines or herbal medicines at the same time
  • have a hormone disorder, such as hypothyroidism
  • have kidney or liver problems

To prevent hypoglycaemia, it's important to have regular meals, including breakfast. Never miss or delay a meal.

If you're planning to exercise more than usual, make sure you eat carbohydrates like bread, pasta or cereals before, during or after exercise.

Always carry a fast-acting carbohydrate with you, like sugar cubes, fruit juice or some sweets, in case your blood sugar level gets low. Artificial sweeteners will not help.

You may also need to eat a starchy carbohydrate, like a sandwich or a biscuit, to maintain your blood sugar for longer.

If taking in sugar does not help or the hypo symptoms come back, contact your doctor or the nearest hospital.

Make sure your friends and family know about your diabetes and the symptoms of low blood sugar levels so they can recognise a hypo if it happens.

Serious allergic reaction

It's possible to have a serious allergic reaction (anaphylaxis) to metformin.

Immediate action required: Call 999 or go to A&E now 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

You could be having a serious allergic reaction and may need immediate treatment in hospital.

These are not all the side effects of metformin. For a full list, see the leaflet inside your medicines packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

6. How to cope with side effects

What to do about:

  • feeling sick – take metformin with food to reduce the chances of feeling sick. It may also help to slowly increase your dose over several weeks. Ask a pharmacist or your doctor for advice.
  • being sick (vomiting) or diarrhoea – drink lots of fluids, such as water or squash, to avoid dehydration. Take small, frequent sips if you're being sick. Speak to a pharmacist if you have signs of dehydration, such as peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea or vomiting without speaking to a pharmacist or doctor.
  • stomach pain – try to rest and relax. It can help to eat and drink slowly and have smaller and more frequent meals. Putting a heat pad or covered hot water bottle on your stomach may also help. If you're in a lot of pain, speak to your pharmacist or doctor.
  • loss of appetite – eat when you'd usually expect to be hungry. If it helps, eat smaller meals more often than usual.
  • a metallic taste in the mouth – if you find that metformin is giving you a metallic taste in the mouth, try chewing sugar-free gum

7. Pregnancy and breastfeeding

Metformin is usually safe to take during pregnancy, either alone or in combination with insulin.

For more information about how metformin can affect you and your baby during pregnancy, read this leaflet on the Best Use of Medicines in Pregnancy (BUMPS) website.

Metformin and breastfeeding

You can take metformin while you're breastfeeding.

Metformin passes into breast milk, but the amount is too small to affect your baby.

Non-urgent advice: Tell your doctor if you're:

  • trying to get pregnant
  • pregnant
  • breastfeeding

8. Cautions with other medicines

There are some medicines that interfere with the way metformin works.

If you're taking any of the following medicines, your blood sugar levels may need to be checked more often and your dose adjusted:

  • steroid tablets, such as prednisolone
  • tablets that make you pee more (diuretics), such as furosemide
  • medicines to treat heart problems and high blood pressure
  • male and female hormones, such as testosterone, oestrogen and progesterone
  • other diabetes medicines

Some women might need a small adjustment in their metformin dose after starting contraceptive pills. That's because contraceptive pills change how your body handles sugar.

Mixing metformin with herbal remedies and supplements

There's very little information about taking herbal remedies and supplements with metformin.

Important: Medicine safety

Tell your doctor or pharmacist if you're taking any other medicines, including herbal medicines, vitamins or supplements.

9. Common questions

How does metformin work in diabetes?

Metformin reduces the amount of sugar your liver releases into your blood.

It also makes your body respond better to insulin.

Insulin is the hormone that controls the level of sugar in your blood.

How does metformin work in PCOS?

Metformin stimulates ovulation in women with PCOS and can encourage regular periods, even if you do not have diabetes.

PCOS cannot be cured, but the symptoms can be managed.

Metformin is not licensed to treat PCOS specifically, but it can sometimes be prescribed to improve fertility. You may have to see a specialist doctor for this.

It'll normally be prescribed when other treatments, such as clomifene, have not worked.

With treatment, most women with PCOS are able to get pregnant.

When will I feel better?

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

This does not mean that metformin is not working, and it's important to keep taking it.

Metformin will help keep your blood sugar level stable and reduce your chances of diabetes-related problems in the future.

How long will I take metformin for?

Treatment for diabetes is usually for life.

Keep taking metformin tablets, unless your doctor tells you to stop.

Can I take metformin for a long time?

Metformin is safe to take for a long time. It will not make you put on weight, and may even help you lose some weight.

It also helps keep your cholesterol at a healthy level.

Your doctor will check how well your kidneys work at least once a year.

You may need more checks if you're an older person or your kidneys are not working normally.

If your kidneys are not working properly, your doctor will tell you to stop taking metformin.

Metformin can cause vitamin B12 deficiency if you take it for a long time.

Your doctor may also check the vitamin B12 level in your blood.

If you become deficient, this can be treated by taking vitamin B12 supplements.

Can I come off metformin?

Do not stop taking metformin without talking to your doctor.

If you stop taking metformin suddenly, you may reduce the control over your diabetes.

Treatment for diabetes is usually for life. But if your kidneys are not working properly, your doctor will tell you to stop taking metformin.

Are there other diabetes medicines?

Metformin is usually the first choice of medicine prescribed to treat type 2 diabetes.

There are other groups of medicines that lower blood sugar levels:

All of these medicines can be prescribed on their own or together with metformin.

Can I get diabetes medicines for free?

If you have diabetes, you're entitled to free prescriptions for all of your medicines, not just your diabetes ones.

To claim your free prescriptions, you'll need to have a medical exemption certificate.

The application form for the medical exemption certificate is called FP92A. You can get this from your doctor's surgery.

You will need to fill in the form, then your doctor will sign it and send it off.

Will it affect my contraception?

Metformin will not affect the combined pill or emergency contraception.

But some women might need a small adjustment in their metformin dose after starting contraceptive pills.

That's because contraceptive pills change how your body handles sugar.

Will it affect my fertility?

There's no clear evidence to suggest that taking metformin will reduce fertility in either men or women.

But speak to a pharmacist or a doctor if you're trying to get pregnant. They may want to review your treatment.

Can I take metformin before surgery?

Your doctor may tell you to stop taking metformin a few days before having an operation or medical tests.

Metformin may interact with the dye used for an X-ray or CT scan.

A general anaesthetic that puts you to sleep can also hide low blood sugar.

Can I drink alcohol with it?

Yes, you can drink alcohol while taking metformin, but it's best for men and women to drink no more than 2 units per day.

Drinking more than this can increase the risk of low blood sugar.

Try to keep to the recommended guidelines of no more than 14 units of alcohol a week.

A standard glass of wine (175ml) is 2 units. A pint of lager or beer is usually 2 to 3 units of alcohol.

Is there any food or drink I need to avoid?

It's a good idea to cut down on foods with added sugar.

Check the nutrition labels, as many foods and drinks are high in sugar, such as:

  • sweets
  • cakes
  • biscuits
  • chocolate
  • some fizzy drinks
  • juice drinks

Be careful eating food and drink containing karela. This is because it can lower your blood sugar levels and mean your diabetes is not controlled as well as it should be.

Karela (also called bitter gourd) is used to flavour foods, such as curries like bitter gourd masala.

It has a bitter taste and is also made into juice and tea.

Can I drive or ride a bike?

If your blood sugar levels are stable, taking metformin should not affect your ability to drive, cycle or use machinery and tools.

Metformin itself will not make your blood sugar levels too low, but your doctor might prescribe it alongside other medicines for diabetes that can affect your blood sugar.

If your blood sugar levels become too low, this can reduce your concentration.

If this happens to you, do not drive, cycle or use machines or tools until you feel better.

Can lifestyle changes help diabetes and PCOS?

There are some lifestyle changes you can make to help control the symptoms of diabetes and PCOS.

These include:

Metformin is usually prescribed when diet and exercise alone has not been enough to control your blood sugar levels.