About trimethoprim

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.

Key facts

Who can and cannot take trimethoprim

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:

How and when to take trimethoprim

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:

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.

Completing your course of trimethoprim

Carry on taking this medicine until you finish the course, even if you feel better. If you stop your treatment early, your problem could come back.

How to take it

You'll usually take trimethoprim twice a day to treat an infection, once in the morning and once in the evening.

You can take it with or without food.

Swallow trimethoprim tablets whole with a drink of water. Do not chew or break them.

Trimethoprim is available as a liquid for people who find it difficult to swallow tablets.

If you're taking trimethoprim as a liquid, shake the bottle gently before measuring out the right amount using a medicine spoon or oral syringe. If you do not have an oral syringe or medicine spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not measure the right amount.

If you're taking trimethoprim to prevent an infection, take it at bedtime.

If you have been prescribed trimethoprim as a treatment for cystitis that comes on after having sex, take it as a single dose within 2 hours of having sex.

How long to take it for

The length of time you'll need to take trimethoprim for depends on how bad and where your infection is, your age, whether you're male or female, and whether you have any other health problems.

Women and children with straightforward UTIs usually take a 3-day course of treatment.

Men and pregnant women with straightforward UTIs usually take a 7-day course of treatment.

People with particularly severe or complicated UTIs, or a catheter, usually take a 14-day course of treatment.

Men with a UTI that causes swelling of the prostate gland (prostatitis) may need a treatment course for 4 to 6 weeks.

People taking it to prevent UTIs may need to take it for at least 6 months.

People taking it for acne may need to take it for at least 6 months.

It's very important to keep taking trimethoprim until your course is finished, even if you feel better. This helps stop the infection coming back.

If you forget to take it

If you forget to take a dose of trimethoprim, take it as soon as you remember, unless it's nearly time for your next dose. In this case, just leave out the missed dose and take your next dose at the usual time.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

If you take too much

Taking an extra dose of trimethoprim is unlikely to harm you, but it may increase the chances of temporary side effects, such as feeling or being sick and diarrhoea.

Contact 111 for advice now if:

You've taken more than your prescribed dose of trimethoprim and:

  • you're worried or get severe side effects
  • you've taken more than 1 extra dose

Go to or call 111

Call 111 if you're asking about a child under the age of 5 years.

Side effects

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:

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:

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

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to trimethoprim.

These are not all the side effects of trimethoprim. For a full list, see the leaflet inside your medicine packet.

How to cope with side effects of trimethoprim

What to do about:

Pregnancy and breastfeeding

Trimethoprim and pregnancy

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.

Cautions with other medicines

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:

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.

Common questions about trimethoprim

How does trimethoprim work? When will I feel better? What if I do not get better? Will it give me thrush? Can I drive or ride a bike? Will it reduce my fertility? Will it stop my contraception working? Can I drink alcohol with it? Is there any food or drink I need to avoid? Does cranberry juice help urinary tract infections? Can lifestyle changes help urinary tract infections?