Type 2 diabetes is a condition 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).
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 is also sometimes used to manage symptoms of polycystic ovary syndrome (PCOS), a condition that affects how the ovaries work. It is not officially approved for PCOS.
Metformin treats PCOS by lowering insulin and blood sugar levels. This can also improve ovulation and encourage regular periods, even if you do not have diabetes.
Metformin is available on prescription as tablets, as a liquid that you swallow and as sachets of powder that you dissolve in a drink.
Who can take metformin
Most adults and children aged 10 years and older can take metformin.
Metformin is not suitable for some people. To make sure it's safe for you, tell your doctor before starting the medicine if you:
- have ever had an allergic reaction to metformin or other medicine
- 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 (given a general anaesthetic)
Dosage and strength
Metformin tablets come in different strengths. Your doctor will tell you how many tablets to take a day.
The maximum daily dose is 2,000mg a day. This can be taken as four 500mg tablets a day.
Liquid metformin should be taken in 5ml doses of 500mg, 850mg or 1,000mg.
Sachets come in either 500mg or 1,000mg doses.
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 tablets, you'll be advised to increase the dose slowly. This reduces the chances of getting side effects.
- one 500mg dose with or after breakfast for at least 1 week, then
- one 500mg dose with or after breakfast and your evening meal for at least 1 week, then
- one 500mg dose with or after breakfast, lunch and your evening meal
If you find that the side effects of standard metformin are affecting you too much, your doctor may suggest switching to slow-release tablets.
How to take it
It's best to take metformin tablets with, or just after, your evening meal to reduce the chance of getting side effects.
Swallow your metformin tablets whole with a drink of water. Do not chew them.
If you're taking metformin sachets, pour the powder into a glass and add water (about 150ml). Stir it if you need to, until the water turns clear or slightly cloudy. Drink the metformin straight away.
How long to take it for
Treatment for diabetes is usually for life. But if your kidneys are not working properly, your doctor will tell you to stop taking metformin and switch you to a different medicine.
Do not stop taking metformin without talking to your doctor.
If you stop taking metformin suddenly, your blood sugar levels will go up and your diabetes will get worse.
If you forget to take it
If you miss a dose of metformin, skip the missed dose and take the next dose at the usual time. Do not take 2 doses 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.
If you take too much
Taking too much metformin can cause serious side effects.
- stomach pain
- fast or shallow breathing
- feeling cold
- unusual sleepiness
- tiredness or weakness
- you take more than your prescribed dose of metformin
If you go to A&E, do not drive yourself. Get someone else to drive you or call for an ambulance.
Take the metformin box or leaflet inside, plus any remaining medicine with you.
Common side effects
These common side effects of metformin happen in more than 1 in 100 people. There are things you can do to help cope with them:
If this advice does not help and any of these side effects continue to bother you, tell your doctor or pharmacist.
Taking metformin can cause vitamin B12 deficiency. Call your doctor or contact 111 if you:
- feel very tired
- have muscle weakness
- have a sore, red tongue
- have mouth ulcers
- have problems with your vision
- have pale or yellow skin (this may be less obvious on brown or black skin
Your doctor can check your vitamin B12 serum levels. If they are too low, they may prescribe B12 vitamin supplements.
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
- 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
- do not eat meals at regular times or skip meals
- are fasting
- do not eat a healthy diet and are not getting enough nutrients
- change what you eat
- exercise too much without eating enough carbohydrates
- drink alcohol, especially after skipping a meal
- take some other medicines or herbal remedies at the same time
- have a hormone disorder, such as an underactive thyroid (hypothyroidism)
- have kidney or liver problems
To prevent hypoglycaemia, it's important to have regular meals, including breakfast. Try not to 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.
Call your doctor or call 111 if taking in sugar does not help, or the hypo symptoms come back
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 side effects
Serious side effects are rare and happen in less than 1 in 10,000 people.
Call your doctor or contact 111 straight away if:
- you get a general feeling of being unwell with severe tiredness, fast or shallow breathing, being cold and a slow heartbeat
- the whites of your eyes turn yellow, or your skin turns yellow, although this may be less obvious on brown or black skin – this can be a sign of liver problems
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to metformin.
Taking metformin can cause vitamin B12 deficiency if you take it for a long time. This can make you feel very tired, breathless and faint, so your doctor may check the vitamin B12 level in your blood.
If your vitamin B12 levels become too low, vitamin B12 supplements will help.
Other side effects
These are not all the side effects of metformin. For a full list, see the leaflet inside your medicine packet.
Metformin and pregnancy
Metformin is safe to take during pregnancy, either alone or combined with insulin.
If your doctor or midwife says your baby is healthy, you can take metformin while breastfeeding.
Metformin passes into breast milk in tiny amounts and has not been linked with side effects in any breastfed babies.
Metformin would not be expected to cause side effects but contact your health visitor, midwife, pharmacist or doctor as soon as possible if your baby:
- is not feeding as well as usual
- seems unusually sleepy or drowsy
- seems unusually restless or irritable
- looks paler, or is more sweaty, than usual
- seems hungrier than usual
- is peeing more
- is causing you any other concerns
Metformin and fertility
There's no evidence to suggest that taking metformin reduces fertility in either men or women.
Metformin is sometimes prescribed to try to improve ovulation and fertility if you have polycystic ovary syndrome (PCOS).
Having a healthy body weight is important both for fertility and for pregnancy. Your doctor may suggest losing weight in the first instance if you have PCOS. This is also likely to improve ovulation and help both you and your baby during pregnancy.
Cautions with other medicines
There are some medicines that can affect 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 changed:
- steroid tablets, such as prednisolone
- tablets that make you pee more (diuretics), such as furosemide
- medicines to treat heart problems and high blood pressure (hypertension)
- male and female hormones, such as testosterone, oestrogen or progesterone
- other diabetes medicines
Your doctor may need to make a small change to your metformin dose if you have just started taking contraceptive pills. This is because contraceptive pills can change how your body handles sugar.
There's not enough information to say that complementary medicines and herbal remedies are safe to take with metformin. They're not tested in the same way as pharmacy and prescription medicines. They're generally not tested for the effect they have on other medicines.
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