Trimethoprim – an antibiotic


Everything you need to know about trimethoprim

What is it Trimethoprim used for?

Trimethoprim is used to treat and prevent recurrent bacterial infections of the urinary tract, bacterial infections of the lungs and airways caused by acute bronchitis, and flare-ups of chronic bronchitis or pneumonia caused by the bacterium Pneumocystis jirovecii.

Key facts about trimethoprim

  • Trimethoprim is suitable for adults and children.
  • The most common side effects of trimethoprim are feeling or being sick, diarrhoea, itching and rash.
  • See a doctor as soon as possible if you get diarrhoea that is severe, persistent or contains blood/mucus.
  • It is OK to drink alcohol while taking trimethoprim.
  • Trimethoprim is not recommended during pregnancy.
  • Trimethoprim is available in tablets (200mg and 100mg) or liquid formulations.

How does trimethoprim work?

Trimethoprim is an antibiotic that works by preventing the bacteria from producing folate. Without folate, the bacteria cannot produce DNA and so are unable to grow and increase in numbers. Trimethoprim therefore stops the spread of infection. The remaining bacteria are killed by the immune system or eventually die.

To make sure the bacteria causing an infection are susceptible to trimethoprim, your doctor may ask you to take a urine or sputum sample.

How do I take trimethoprim?

The dose and duration of trimethoprim prescribed depends on the type and severity of infection and your kidney function.

  • Trimethoprim is usually taken twice a day (every 12 hours) to treat infections and once a day in the evening to prevent urinary tract infections.
  • Trimethoprim can be taken either with or without food.
  • Take the medicine at regular intervals. If you miss a dose take it as soon as you remember. Don’t take a double dose to make up for a missed dose.
  • Unless your doctor tells you otherwise, it is important that you finish the prescribed course of this antibiotic medicine, even if you feel better or it seems the infection has cleared up. Stopping the course early increases the chances of the infection coming back and bacteria can grow resistant to the antibiotic.

Who should not take trimethoprim?

  • Pregnant women.
  • People having problems with blood cell counts. Trimethoprim may very rarely cause a decrease in the normal amounts of blood cells in the blood. For this reason, you should consult your doctor immediately if you experience any of the following symptoms: unexplained bruising or bleeding, purple spots, sore throat, mouth ulcers, high temperature.
  • People with decreased kidney or liver function may need to take a lower dose or have extra monitoring if taking trimethoprim.
  • Trimethoprim should be used with caution in people at risk of folic acid deficiency.

Can I use trimethoprim while pregnant or breastfeeding?

  • Trimethoprim should be avoided during pregnancy, particularly during the first trimester, as it may be harmful to the developing baby.
  • Trimethoprim is not known to be harmful if used short-term. Your doctor will be able to advise you.

Side effects of trimethoprim

Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with trimethoprim. Just because a side-effect is stated here, it does not mean that all people using this medicine will experience it, or any side-effect.

Common side effects:

  • Nausea and vomiting.
  • Overgrowth of the yeast Candida, which may cause infection such as thrush.

Other side effects of trimethoprim include:

  • Skin reactions, such as rash, hives, itching and photosensitivity. See your doctor if you get a rash.
  • Inflammation of the bowel (colitis). Tell your doctor if you get diarrhoea that is severe, persistent or contains blood or mucus.
  • Liver problems. Tell your doctor if you experience yellow skin or eyes (jaundice).
  • Problems with your blood cells. Tell your doctor if you experience unexplained bruising, sore throat, fever or infections.
  • Increased levels of potassium in the blood (hyperkalaemia).

Read the leaflet that comes with the medicine or talk to your doctor or pharmacist if you want any more information about the possible side effects of trimethoprim. If you think you have experienced a side effect, you can report it using the yellow card website.

Before you take trimethoprim, tell your doctor or pharmacist if you’re already taking any medicines, including those bought without a prescription and herbal medicines. Always check with your doctor or pharmacist before taking any new medicines while you’re taking trimethoprim.

Here is a list of the key medicines that may interact with trimethoprim:

  • Rifampicinmay reduce the blood level of trimethoprim and could potentially make it less effective.
  • The anti-blood-clotting effect of warfarinmay be increased if taken in combination with trimethoprim.
  • Trimethoprim can increase the amount of digoxinand phenytoin in the blood. This may increase the risk of side effects associated with digoxin and phenytoin.
  • Trimethoprim may enhance the blood sugar lowering effect of repaglinide and pioglitazone.
  • Oral typhoid vaccine (Vivotif) should not be taken until at least three days after you have finished a course of trimethoprim, because the antibiotic could make the vaccine less effective.


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