Like all medicines, phenoxymethylpenicillin can cause side effects, although not everyone gets them.
If you're taking phenoxymethylpenicillin to prevent infections, your doctor will usually ask you to have blood tests. This is to make sure you're not getting any unexpected side effects.
Common side effects
These common side effects of phenoxymethylpenicillin happen in 1 in 10 people. There are things you can do to help cope with them:
Feeling sick (nausea)
Stick to simple meals and do not eat rich or spicy food while you're taking this medicine.
Being sick (vomiting)
Take small, frequent sips of 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 vomiting without speaking to a pharmacist or doctor.
If you take the combined contraceptive pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
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 the combined contraceptive 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 doctor.
Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Mild allergic reactions
Around 1 in 10 people have an allergic reaction to phenoxymethylpenicillin.
In most cases, the allergic reaction is mild.
You can usually treat a mild allergic reaction by taking antihistamines.
Talk to your doctor or pharmacist if you have a mild allergic reaction and your symptoms get worse.
Serious side effects
Serious side effects are rare and happen in less than 1 in 1,000 people.
Call your doctor or call 111 now if you get:
- stomach pain
- a high temperature and watery diarrhoea with blood and mucus in it – this could be a sign that your bowels have become irritated and swollen (inflamed).
Serious allergic reaction
In rare cases, phenoxymethylpenicillin can cause a serious allergic reaction (anaphylaxis).
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.
Other side effects
These are not all the side effects of phenoxymethylpenicillin. For a full list, see the leaflet inside your medicine packet.
You can report any suspected side effect using the Yellow Card safety scheme.