Metformin can cause side effects, but not everyone gets them.
Stop taking metformin and talk to your GP or pharmacist if side effects bother you or do not go away.
Common side effects
There are things you can do to manage common side effects:
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 GP for advice.
Getting 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 medicines to treat vomiting without speaking to a pharmacist or GP.
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 medicines to treat diarrhoea without speaking to a pharmacist or GP.
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 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 GP.
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 your mouth
Try chewing sugar-free gum.
Serious side effects
Serious side effects are rare. But they can happen.
Emergency action required: Call 112 or 999 or go to an emergency department (ED) if you have:
- severe tiredness
- difficulty breathing
- feeling cold
- a slow heartbeat
- vomiting or tummy pain
These may be signs of a rare but serious condition called lactic acidosis.
Lactic acidosis happens when there is too much lactic acid in your blood.
Urgent advice: Contact your GP and stop taking metformin immediately if you:
- have yellowing of your skin or whites of your eyes - this can be a sign of liver problems
- have extreme tiredness, pins and needles, sore and red tongue, muscle weakness and changes in your vision - these could be signs of vitamin B12 deficiency anaemia
- have a rash, redness or itching on skin
- dehydration that may be caused by severe vomiting, diarrhoea, fever, exposure to heat or you drink less fluid than normal - this could increase the risk of lactic acidosis
Vitamin B12 deficiency
Taking metformin can cause vitamin B12 deficiency.
Urgent advice: Call your GP 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
Your GP can check your vitamin B12 levels. If they are too low, they may prescribe B12 supplements.
Risk of lactic acidosis
Lactic acidosis is a very rare and serious side effect of metformin. It can be worse if your kidneys are not working properly.
You are at greater risk of getting lactic acidosis if you:
- have diabetes that is not controlled
- have a serious infection
- fast for long periods
- drink a lot of alcohol
- are dehydrated
- have liver or kidney problems
- 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.
Serious allergic reaction
A serious allergic reaction (anaphylaxis) to metformin is rare, but it can happen.
Emergency action required: Phone 112 or 999 or go to an ED immediately if you have:
- a rash that is itchy, red, swollen, blistered or peeling
- wheezing
- tightness in your chest or throat
- trouble breathing or talking
- swelling of your mouth, face, lips, tongue or throat
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 Health Products Regulatory Authority (HPRA): report an issue - hpra.ie