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.

• 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.

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

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

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.

These are not all the side effects of metformin.

For a full list, see the leaflet inside your medicines packet.

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

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.

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.