Like all medicines, metformin can cause side effects, although not everyone gets them.
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:
Feeling sick (nausea)
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)
Take small, frequent sips of water or squash to avoid dehydration. 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 vomiting without speaking to a pharmacist or doctor.
If you take the combined pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet to find out what to do.
Diarrhoea
Drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
If you take combined pill or progestogen-only pill and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Stomach ache
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
Try chewing sugar-free gum.
If this advice does not help and any of these side effects continue to bother you, tell your doctor or pharmacist.
Vitamin B12 deficiency
Taking metformin can cause vitamin B12 deficiency. Call your doctor or call 111 straight away 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
- 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
- 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 call 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.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Long-term side effects
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.
Reporting side effects
You can report any suspected side effect using the Yellow Card safety scheme.
Visit the Yellow Card website for more information about reporting side effects.