Like all medicines, cefalexin can cause side effects in some people, but many people have no side effects or only minor ones.
Common side effects
These common side effects of cefalexin happen in around 1 in 10 people. There are things you can do to help cope with them:
Feeling sick (nausea)
Eat simple meals and do not eat rich or spicy food while you're taking this medicine. It might help to take your cefalexin after a meal or snack.
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. If your diarrhoea continues, speak 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. Try putting a heat pad or covered hot water bottle on your stomach.
Keep taking the medicine, but talk to your doctor or pharmacist if the advice on how to cope does not help and these side effects bother you or do not go away.
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:
- severe diarrhoea or there is blood or mucus in your diarrhoea
- pale poo and dark pee, yellowing of your skin (this may be less obvious on brown or black skin), or whites of your eyes – this may be a sign of liver problems
- unexplained bruising on your skin
Serious allergic reaction
In rare cases, cefalexin may 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.
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Other side effects
These are not all the side effects of cefalexin. 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.