Co-trimoxazole for infection - Septrin

Co-trimoxazole is a combination antibiotic. It can be taken by people who are allergic to penicillin, but it is not suitable for people who are allergic to sulfonamides.

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

The most common side-effects are feeling sick, diarrhoea and headache. If you develop a skin rash or any allergic-type reaction, let your doctor know straightaway.

Type of medicine An antibacterial medicine
Used for To treat or prevent certain infections
Also called Septrin®
Available as Tablet, oral liquid medicine and injection

Co-trimoxazole is a combination of two antibacterial medicines - a sulfonamide medicine called sulfamethoxazole, and trimethoprim. Although it has been prescribed widely for a range of infections in the past, it has very occasionally been associated with some serious side-effects. As a result, other antibiotics are now preferred to treat simple infections and co-trimoxazole is usually reserved for the treatment of more serious infections. In particular, it is prescribed for infections which can occur in people who have a problem with their immune systems. It works by killing the germs (bacteria) responsible for causing the infection.

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 co-trimoxazole it is important that your doctor knows:

  • If you are pregnant, trying for a baby or breast-feeding.
  • If you have any problems with the way your liver works or with the way your kidneys work.
  • If you have asthma.
  • If you have a blood disorder. It is particularly important that you tell your doctor if you have a rare inherited blood disorder called porphyria.
  • If you have an enzyme deficiency called G6PD deficiency.
  • If you have been told you have low amounts of the vitamin folic acid.
  • 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. It is particularly important that you tell your doctor if you have ever had a bad reaction to a sulfonamide antibiotic.
  • Before you start the treatment, read the manufacturer's printed information leaflet from inside the pack. It will give you more information about co-trimoxazole and will provide you with a full list of the side-effects which you may experience from taking it.
  • Take co-trimoxazole 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, it is usual to take two doses of co-trimoxazole daily (12 hours apart) to treat an infection, but make sure you read the directions on the label, as your dose may be different to this. If co-trimoxazole has been prescribed for your child, it is likely that you will be supplied a liquid medicine - the dose will depend upon the age of the child. Read the directions on the label carefully and make sure you measure out the correct dose.
  • Remember to take co-trimoxazole regularly so that you keep a steady level of the medicine in your body. Try to avoid missing any doses.
  • Co-trimoxazole is best taken with food or a drink.
  • 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.
  • Try to drink several glasses of water each day while you are on co-trimoxazole. It is important that you do not become dehydrated.
  • Most people improve within a few days of starting treatment. If your symptoms do not improve despite taking co-trimoxazole, 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.
  • Even if you feel your infection has cleared up, keep taking the antibiotic until the course is finished (unless your doctor tells you otherwise). This is to prevent the infection from coming back and being more difficult to treat. A typical course of treatment will last for 5-7 days, although the treatment of some of the more serious infections can take longer than this.
  • Some people develop thrush (a yeast infection which causes redness and itchiness in the mouth or vagina) after taking a course of antibiotics. If you think you have thrush speak to your doctor or pharmacist for advice.
  • If you are having an operation or dental treatment, tell the person carrying out the treatment which medicines you are taking.
  • If you buy any medicines, check with a pharmacist that they are suitable to take with co-trimoxazole.
  • This medicine may 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.
  • If you still feel unwell after completing your course of the antibiotic, make another appointment to see your doctor.

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 co-trimoxazole. 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.

Common co-trimoxazole side-effects What can I do if I experience this?
Feeling sick Stick to simple foods - avoid rich or spicy foods
Diarrhoea Drink plenty of water to replace any lost fluids
Headache Ask your pharmacist to recommend a suitable painkiller
Skin rash Contact your doctor for advice straightaway. Do not take any more doses until you have spoken with a doctor

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

  • 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. 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

  • Manufacturer's PIL, Co-Trimoxazole Tablets 80/400 mg; Actavis UK Ltd, The electronic Medicines Compendium. Dated December 2012.
  • British National Formulary; 67th Edition (March 2014) British Medical Association and Royal Pharmaceutical Society of Great Britain, London

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:
Dr Adrian Bonsall
Document ID:
3392 (v23)
Last Checked:
22/08/2014
Next Review:
21/08/2017
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