Trimethoprim is an antibiotic.

It's used to treat water infections, such as cystitis.

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

Trimethoprim is available on prescription as tablets and as a liquid that you drink.

  1. Key facts

    • Trimethoprim is usually taken twice a day to treat infections.
    • For most infections, you'll feel better within a few days.
    • Side effects may include itching or a mild skin rash, but are usually mild and short-lived.
    • You can drink alcohol while taking trimethoprim.
    • There are no brand names for this medicine at the moment.
  2. Who can take trimethoprim

    Trimethoprim can be taken by adults and children.

    Trimethoprim isn't 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
    • 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 you're already pregnant or breastfeeding
  3. How and when to take

    Trimethoprim is usually taken twice a day to treat an infection – once in the morning and once in the evening. You can take it with or without food.

    The usual dose of trimethoprim to:

    • treat water infections 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 one-off dose of 100mg
    • 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 illness, your age, and how well your kidneys work. Doses are usually lower for elderly people and those with kidney problems.

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

    How to take it

    Swallow trimethoprim tablets whole with a drink of water. Don't 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, it'll usually be made up for you by your pharmacist. The medicine will come with a syringe or spoon to help you take the right amount. If you don't have a syringe or spoon, ask your pharmacist for one.

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

    If you've 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 will I 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 with straightforward water infections usually take a 3-day course of treatment.
    • Men and pregnant women with straightforward water infections usually take a 14-day course of treatment.
    • People with particularly severe or complicated water infections, or a catheter, usually take a 14-day course of treatment.
    • A treatment course for 4 to 6 weeks could be needed if the water infection causes swelling of the prostate gland in men (prostatitis).
    • Treatment may continue for at least 6 months for preventing water infections or as a treatment for acne.

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

    What if I forget to take it?

    If you forget to take a dose, take one 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 as normal.

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

    If you forget doses often, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways that are suitable for you and your medicines.

    What if I take too much?

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

    Talk to your doctor or pharmacist if you:

    • are worried or get severe side effects
    • have taken more than one extra dose
  4. 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

    The most common side effects with trimethoprim are itching or a mild rash. They happen in more than 1 in 100 people.

    Trimethoprim can also make you feel sick or have diarrhoea. It may also cause headaches.

    Serious side effects

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

    Tell a doctor straight away if you have:

    • muscle weakness, being unable to move or feel anything, an abnormal heartbeat, chest pains, and feeling sick (warning signs of high potassium in your blood)
    • muscle cramps, tiredness, feeling or being sick, feeling confused and headaches (warning signs of salt in your blood)
    • serious skin reactions or rashes, including irregular, round red patches, peeling, blisters, skin ulcers, or swelling of the skin that looks like burns
    • headache, fever, stiff neck, tiredness, feeling ill, and your eyes becoming very sensitive to bright light (warning signs of meningitis)
    • diarrhoea, possibly with stomach cramps, is severe, contains blood or mucus, or lasts longer than 4 days
    • bruising or bleeding you can't explain (including nosebleeds), a sore throat, mouth ulcers, a high temperature, or feeling tired or generally unwell (warning signs of a problem with your blood)

    Serious allergic reactions

    The warning signs of a serious allergic reaction are:

    • difficulty breathing
    • swelling of the face, lips or throat
    • severe skin reactions, including large, fluid-filled blisters, sores and ulcers
    • ulcers in the throat and mouth
  5. How to cope with side effects

    What to do about:

    • itching or a mild rash – try rubbing a gentle, fragrance-free skin moisturiser over the affected skin. It may also help to take an antihistamine, which you can buy from a pharmacy (check with the pharmacist to see what type is suitable for you).
    • feeling sick – try taking trimethoprim with or after food to see if that helps ease feelings of sickness. It may also help if you avoid rich or spicy food while you're taking this medicine.
    • diarrhoea and vomiting – drink plenty of water or other fluids if you have diarrhoea or you're being sick. It may also help to take oral rehydration solutions you can buy from a pharmacy to prevent dehydration. Don't take any other medicines to treat diarrhoea or vomiting without speaking to a pharmacist or doctor.
    • headaches – make sure you rest and drink plenty of fluids. Don't drink too much alcohol

    Painkillers you can buy, such as paracetamol and ibuprofen, are safe to take with trimethoprim. Talk to your doctor if these don't help with the headaches or you have other symptoms, particularly those of meningitis.

    You can report any suspected side effect to a UK safety scheme.

  6. Pregnancy and breastfeeding

    Trimethoprim isn't the safest antibiotic to take in pregnancy. Doctors generally agree you should take it only if the benefits outweigh the risks. It's been linked with a small risk of problems for the unborn baby if it's taken in early pregnancy.

    A substance called folic acid is important for the normal development of an unborn baby. Pregnant women are routinely advised to take a 400mcg folic acid supplement every day for the first 12 weeks.

    Trimethoprim lowers levels of folic acid in the bloodstream. If you take this medicine during the first 12 weeks of pregnancy, your doctor will probably prescribe a high dose of folic acid (5mg daily) for you to take along with the trimethoprim.

    There are no known risks to a pregnant woman or her unborn baby from taking trimethoprim after the first 12 weeks.

    You can breastfeed as normal while taking trimethoprim. Trimethoprim passes into breast milk, but only in tiny amounts that aren't harmful to the baby.

    Talk to your pharmacist or doctor if you're worried.

    For safety, tell your doctor if you're pregnant or become pregnant while taking this medicine.

  7. Cautions with other medicines

    There are many medicines that don't mix well with trimethoprim.

    Tell your doctor if you're taking these medicines before starting trimethoprim:

    • an antibiotic called rifampicin
    • a blood thinner, such as warfarin
    • digoxin (a heart medicine)
    • phenytoin (an epilepsy medicine)
    • diabetes medicines called replaglinide and pioglitazone

    Typhoid vaccine given by mouth may not work properly if you're taking trimethoprim. This doesn't apply to typhoid vaccines given by injection.

    Mixing trimethoprim with herbal remedies and supplements

    There are no known problems with taking herbal remedies and supplements with trimethoprim.

    Tell your doctor or pharmacist if you're taking any other medicines, including herbal remedies and supplements.

  8. Common questions