Skip to main content

Trimethoprim for infections

Trimethoprim is an antibacterial medicine prescribed for treating infections, mainly chest or urine infections. It is sometimes prescribed to prevent an infection.

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

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

Continue reading below

About trimethoprim

Type of medicine

An antibacterial medicine

Used for

Bacterial infections (in adults and children)

Available as

Tablets and oral liquid medicine

Trimethoprim is given to treat a bacterial infection. It is mainly prescribed for urine infections, but it is also prescribed for chest infections and some other types of infection. It works by killing the germs (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 the infection from recurring.

Before taking trimethoprim

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

  • If you are pregnant, trying for a baby or breastfeeding.

  • 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 problems with the way your liver works.

  • If you have a blood disorder, or if you have a rare inherited blood disorder called porphyria.

  • If you are taking any other medicines. This includes any medicines which are available to buy without a prescription, as well as herbal and complementary medicines.

  • If you have ever had an allergic reaction to a medicine.

Continue reading below

How to take trimethoprim

  • Before you start the treatment, read the manufacturer's printed information leaflet from inside the pack. It will give you more information about trimethoprim and will provide you with a full list of the side-effects which you could 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, taken 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. Swallow the tablet with a drink of water. You can take trimethoprim either with or without food.

  • Even if you feel your infection has cleared up, keep taking trimethoprim until the full course is finished (unless a doctor tells you to stop). This is to prevent the infection from coming back. A normal course of treatment is likely to last 3-14 days. The course will be for longer than this if you are taking trimethoprim to prevent recurrent infections.

  • If you forget to take a dose at the correct time, take one as soon as you remember. Try to take the correct number of doses each day. Do not take two doses at the same time to make up for a forgotten dose.

Getting the most from your treatment

  • 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 medicine. This is because some bacteria are resistant to some types of antibiotics.

  • If you are taking the contraceptive 'pill' at the same time as this antibacterial medicine, the effectiveness of the 'pill' can be reduced if you have a bout of being sick (vomiting) or diarrhoea which lasts for more than 24 hours. If this should happen, ask your doctor or pharmacist for advice about what additional contraceptive precautions to use over the following few days. There is no need to use additional precautions for any bouts of sickness or diarrhoea which last for less than 24 hours.

  • Trimethoprim can stop the oral typhoid vaccine from working. If you are due to have any vaccinations, make sure the person treating you knows that you are taking it.

Continue reading below

Can trimethoprim cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with trimethoprim. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Very common trimethoprim side-effects (these affect more than 1 in 10 people)


What can I do if I experience this?

Feeling sick (nausea) or being sick (vomiting)

Stick to simple foods - avoid fatty or spicy meals. If you are not already doing so, try taking trimethoprim after food

Common trimethoprim side-effects (these affect fewer than 1 in 10 people)

What can I do if I experience this?


Drink plenty of water to replace lost fluids. Stick to simple foods


Drink plenty of water and ask your pharmacist to recommend a suitable painkiller. If the headaches continue, let your doctor know

Itchy skin rash

Try 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 high temperature (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.

How to store trimethoprim

  • Keep all medicines out of the reach and sight of children.

  • Store in a cool, dry place, away from direct heat and light.

Important information about all medicines

Important information about all medicines

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.

If you buy any medicines, check with a pharmacist that they are safe to take with your other medicines.

If you are having an operation or any dental treatment, tell the person carrying out the treatment which medicines you are taking.

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.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

symptom checker

Feeling unwell?

Assess your symptoms online for free