Trimethoprim for infections

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Trimethoprim is an antibiotic used mainly for treating chest or urine infections. It is sometimes taken to prevent infections.

Space out your doses evenly throughout the day, and remember to finish the course of treatment.

The most common side-effects are feeling sick, and a mild itchy rash.

Type of medicineAntibiotic
Used forBacterial infections
Also calledMonotrim®
Available asTablets and oral liquid

Trimethoprim is used to treat infections which are caused by bacteria, such as some chest infections and urine infections. It works by killing the bacteria responsible for the infection.

A urine infection is often called a urinary tract infection (UTI) by doctors. Most urine infections are caused by bacteria that come from your own bowel. They are usually easily treated with a short course of trimethoprim. Occasionally, longer-term treatment may be needed to prevent infections from coming back.

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking trimethoprim it is important that your doctor or pharmacist knows:

  • If you are pregnant, trying for a baby or breast-feeding.
  • If you know you have low amounts of the vitamin folic acid.
  • If you have problems with the way your kidneys work.
  • If you have a blood disorder, or a condition known as porphyria (this is a rare inherited blood disorder).
  • If you are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, such as herbal and complementary medicines.
  • If you have ever had an allergic reaction to a medicine.
  • Before you start this treatment, read the manufacturer's printed information leaflet from inside your pack. The leaflet will give you more information about trimethoprim and a full list of side-effects which you may experience from taking it.
  • Take trimethoprim tablets exactly as your doctor tells you to. Your dose will depend upon whether you are being treated because you have an infection, or to prevent an infection. As a guide, the dose for adults with an infection is 200 mg twice daily (morning and evening). The adult dose to prevent an infection is 100 mg at night. If trimethoprim has been prescribed for a child, it is likely that you will be supplied a liquid medicine - the dose will depend upon the age of the child, so read the directions on the label carefully to make sure you give the correct dose.
  • Space your doses out evenly throughout the day - this means that tablets/medicine prescribed twice daily should be taken every 12 hours. You can take trimethoprim before or after meals.
  • Keep taking this antibiotic until the course is finished, unless you are told to stop. A normal course of treatment is likely to last 3-14 days. It is important that you do this even if you feel well, otherwise your infection may come back.
  • If you forget to take a dose, take it as soon as you remember. If it is almost time to take your next dose when you remember, skip the missed dose and take your next dose when it is due. Do not take two doses together to make up for a forgotten dose.
  • Most people improve within a few days of starting treatment. If your symptoms do not improve despite taking trimethoprim, go back to see your doctor, as you may need an alternative antibiotic. This is because some bacteria are resistant to some types of antibiotics.
  • If you buy any medicines, check with a pharmacist that they are suitable to take with trimethoprim.
  • If you are using oral combined hormonal contraception (the 'pill'), the current recommendation is that additional contraceptive precautions such as condoms are not required during a course of this antibiotic, unless you are sick or have diarrhoea. If you need further advice about this, speak with your doctor or pharmacist.
  • If you are having an operation or dental treatment, tell the person carrying out the treatment that you are taking an antibiotic.
  • This medicine may stop the oral typhoid vaccine from working. If you are having any vaccinations, make sure the person treating you knows that you are taking it.

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. These usually improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following side-effects continue or become troublesome.

The most common trimethoprim side-effects are:
What can I do if I experience this?
Feeling or being sickStick to simple foods - avoid rich or spicy meals. If you are not already doing so, try taking trimethoprim after food
Itchy skin rashTry applying a gentle, fragrance-free skin moisturiser. If the rash becomes troublesome or severe, speak with your doctor

Important: If trimethoprim is taken over a long period of time (such as when it is taken long-term to prevent an infection), it can occasionally cause problems. If you develop a fever, sore throat, skin rash, mouth ulcers or any bruising or bleeding which you can't explain, you should let your doctor know about this straightaway. These may be signs of a blood disorder.

If you experience any other symptoms which you think may be due to this medicine, speak with your doctor or pharmacist.

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital at once. Take the container with you, even if it is empty.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Further reading & references

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Helen Allen
Current Version:
Peer Reviewer:
Prof Cathy Jackson
Document ID:
3344 (v23)
Last Checked:
Next Review:
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