Like all medicines, trimethoprim can cause side effects in some people, but many people have no side effects or only minor ones.
Common side effects of trimethoprim
These common side effects of trimethoprim happen to more than 1 in 100 people. There are things you can do to help cope with them:
Itching or a mild rash
It may help to use an emollient cream or gentle, fragrance-free moisturising cream, or take an antihistamine, which you can buy from a pharmacy. Check with a pharmacist to see what may be suitable for you.
Feeling or being sick (nausea or vomiting)
Try taking trimethoprim with or after food to see if that helps your symptoms. It may also help if you stick to simple meals and avoid rich or spicy food while you're taking this medicine.
If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
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 to find out what to do.
Diarrhoea
Drink lots of water or squash to avoid dehydration. Speak to a doctor or 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 diarrhoea without speaking to a pharmacist or doctor.
If you take the combined contraceptive pill or progestogen-only pill and you have severe, watery diarrhoea or any diarrhoea that lasts for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet to find out what to do.
Thrush
Trimethoprim can sometimes cause the infection, thrush. Drying properly after washing, wearing cotton underwear and avoiding sex until it's cleared up can help.
Headaches
Make sure you rest and drink plenty of fluids. It's best not to drink too much alcohol. Everyday painkillers, such as paracetamol and ibuprofen, are safe to take with trimethoprim. Speak to your doctor if these do not help or the headaches are severe and do not go away.
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.
Serious side effects
Most serious side effects are rare and happen in less than 1 in 1,000 people.
Call a doctor or call 111 straight away if:
- you have diarrhoea that contains blood or mucus, or severe diarrhoea
- you have sudden, severe pain in your stomach, and may be feeling or being sick (nausea and vomiting) – these can be signs of pancreatitis
- 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
- you get bruising or bleeding you cannot explain (including nosebleeds), a sore throat, mouth ulcers, a high temperature, or you feel tired or generally unwell – these can be signs of a problem with your blood
- you have serious skin reactions or rashes, including, round red patches, peeling, blisters, skin ulcers, or swelling of the skin that look like burns
Immediate action required: Call 999 or go to A&E now if:
- you have headaches, a high temperature, a stiff neck, tiredness, feel ill, and your eyes become very sensitive to bright light – these can be signs of meningitis which is a very rare side effect of trimethoprim
- you have muscle weakness, an abnormal heartbeat or chest pains – these can be signs of high potassium in your blood
Do not drive to A&E. Ask someone to drive you or call 999 and ask for an ambulance.
Bring all the medicines you take with you.
Serious allergic reaction
In rare cases, trimethoprim 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 trimethoprim. 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.