Trimethoprim is an antibiotic.

It's used to treat and prevent urinary tract infections (UTIs), such as cystitis.

Occasionally, trimethoprim is used to treat other types of infections, such as chest infections and acne.

Trimethoprim is available on prescription. It comes as tablets and as a liquid that you swallow.

• You'll usually take trimethoprim twice a day to treat infections.
• For most infections, you'll feel better within a few days. But it's important to finish your course of trimethoprim to help stop your infection from coming back.
• Side effects may include itching or a skin rash, but these are usually mild and short-lived.
• You can drink alcohol while taking trimethoprim.
• Tell your doctor if you do not start feeling better after taking trimethoprim for 3 days, or if you start to feel worse at any time.

Most adults and children can take trimethoprim.

Trimethoprim is not suitable for some people. To make sure this medicine is safe for you, tell your doctor if you:

• have ever had an allergic reaction to trimethoprim or any other medicine
• have liver or kidney problems
• have anaemia or low amounts of folic acid (folate) in your blood
• have porphyria (a rare inherited blood disorder) or any other blood disorder
• are trying to get pregnant or are already pregnant

Always follow your doctor or pharmacist's advice, and the instructions that come with your medicine.

Dosage and strength

Trimethoprim tablets contain 100mg or 200mg. The liquid contains 50mg in 5ml.

The usual dose of trimethoprim to:

• treat urinary tract infections (UTIs) is 200mg twice a day – your doctor might recommend you double the first dose to 400mg
• prevent infections is 100mg once a day
• treat cystitis that comes on after having sex is a single dose of 100mg taken within 2 hours of having sex
• treat acne is 300mg twice a day – this dose might be reduced over time

The dose of trimethoprim you need to take depends on your condition, your age, and how well your kidneys work.

Doses are usually lower for children, older people and those with kidney problems.

You're unlikely to get side effects from trimethoprim. Some people get itching or a skin rash, but this is usually mild and goes away after you stop taking the medicine.

Common side effects

These common side effects of trimethoprim happen in more than 1 in 100 people. Speak to your doctor or pharmacist if these side effects bother you or do not go away:

• itching or a mild rash
• feeling or being sick (nausea or vomiting)
• diarrhoea
• headaches

Serious side effects

Most serious side effects are rare and happen in less than 1 in 1,000 people.

Call a doctor or contact 111 straight away if you have:

• diarrhoea (possibly with stomach cramps) that contains blood or mucus, or severe diarrhoea that lasts longer than 4 days
• 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

What to do about:

• itching or a mild rash – it may help to use an emollient cream or gentle, fragrance-free moisturiser, or take an antihistamine, which you can buy from a pharmacy. Check with the pharmacist to see what may be suitable for you.
• feeling or being sick – try taking trimethoprim with or after food to see if that helps ease 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.
• diarrhoea – drink plenty of water or squash to avoid dehydration. Speak to a doctor if you have signs of dehydration such as peeing less than usual or having dark, strong-smelling pee. It may also help to take oral rehydration solutions which you can buy from a pharmacy. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
• 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.

Trimethoprim can be taken in pregnancy, but it is not recommended in the first 12 weeks. This is because it may affect your levels of folic acid, which is important in the early stages of your baby's development.

Sometimes trimethoprim is the only suitable antibiotic to treat an infection. In this case the benefit of taking it is likely to outweigh the small risk of harm, but discuss this with your doctor.

If you do need to take trimethoprim while you're trying to get pregnant or during the first 12 weeks of pregnancy, your doctor may also recommend taking high dose folic acid (5mg per day).

It's OK to take trimethoprim in later pregnancy (after 12 weeks) and there's no particular need to take high dose folic acid with it, although it will not do any harm if you continue to take it.

Trimethoprim and breastfeeding

If your doctor or health visitor says your baby is healthy, you can take trimethoprim while breastfeeding.

Trimethoprim passes into breast milk in small amounts and is unlikely to cause side effects in your baby. But it's best to take trimethoprim only for a short time because it may affect your baby's folic acid levels. If you need to take trimethoprim for longer than a few weeks, talk to your doctor or pharmacist.

If your baby is not feeding as well as usual, has a stomach upset, or has oral thrush (a fungal infection in their mouth), or if you have any other concerns about your baby, talk to your doctor, pharmacist, health visitor or midwife.

For more information about how trimethoprim can affect you and your baby during pregnancy, visit the Best Use of Medicines (BUMPS) website.

There are many medicines that do not mix well with trimethoprim.

Tell your doctor or pharmacist if you're taking any other medicines before starting to take trimethoprim, in particular:

• rifampicin, an antibiotic
• warfarin or acenocoumarol (Sinthrome), medicines to treat or prevent blood clots (anticoagulants)
• digoxin, a heart medicine
• phenytoin, an epilepsy medicine
• replaglinide or pioglitazone, diabetes medicines

Typhoid vaccines given by mouth may not work properly if you're taking trimethoprim. It does not affect typhoid vaccines given by injection.

Mixing trimethoprim with herbal remedies and supplements

It's not possible to say that complementary medicines or herbal remedies are safe to take with trimethoprim. They're not tested in the same way as pharmacy and prescription medicines.