Sulfamethoxazole with trimethoprim
Co-trimoxazole
A combination antibiotic used mainly to treat and prevent PCP pneumonia and some other specific infections.
What is Co-trimoxazole?
Co-trimoxazole is a combination antibiotic that pairs two medicines, sulfamethoxazole and trimethoprim, which work together to block the growth of bacteria. In the UK it is used mainly to treat and to prevent a type of pneumonia called PCP (Pneumocystis pneumonia) in people whose immune system is weakened, and for a few other specific infections where it is the right choice. It is not a first-choice antibiotic for everyday infections, partly because it can occasionally cause serious skin or blood reactions. People with a sulfonamide (sulfa) allergy should not take it, and your blood counts and kidney function may be checked during treatment.
Education and reference only. This is a plain-language guide to Co-trimoxazole — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Co-trimoxazole is a fixed combination of two antibiotics, sulfamethoxazole and trimethoprim, taken together because they act on the same bacterial pathway from two angles. In the UK it has a fairly narrow role: it is used above all to treat and to prevent PCP (Pneumocystis pneumonia), a serious chest infection that mainly affects people with a weakened immune system, such as some people living with HIV or those on certain immune-suppressing treatments. It is also used for a handful of other specific infections, such as some that are caused by bacteria resistant to other antibiotics, when a specialist or the local guidance recommends it. It is taken as tablets or a liquid, and is sometimes given by drip in hospital.
How it works
The two ingredients block different steps that bacteria use to make folate, a substance they need to build new DNA and multiply. Sulfamethoxazole blocks one step and trimethoprim blocks a later one, so together they squeeze the same pathway harder than either would alone, which is why they are combined. This dual action makes co-trimoxazole effective against the organism that causes PCP and against certain other bacteria. Because human cells get their folate ready-made from the diet rather than building it this way, the medicine targets the microbes more than the body, though it can still affect folate-dependent processes such as making blood cells.
Company & origin
Originated / developed by: Generic (long-established).
A long-established combination antibiotic used in the UK mainly to treat and prevent a particular lung infection in people with weakened immunity.
Practical use
How to take Co-trimoxazole
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it exactly as prescribed, usually twice a day, and try to space the doses evenly through the day.
- Swallow tablets with a full glass of water and keep your fluid intake up while you are taking it.
- Finish the whole course as directed, even if you feel better, unless your prescriber tells you to stop.
- Stop the medicine and seek urgent medical advice if you develop a spreading rash, blisters, peeling skin, mouth ulcers or a sore throat with fever.
- Tell your prescriber and pharmacist about all your other medicines, as co-trimoxazole interacts with several, including some used for the heart, blood pressure and blood thinning.
Weighing it up
Advantages & disadvantages of Co-trimoxazole
Advantages
- A well-targeted and effective choice for treating and preventing PCP pneumonia in people with weakened immunity.
- Two antibiotics in one work together on the same pathway, which can also cover some bacteria resistant to other antibiotics.
- Available as tablets and liquid, making it usable at home as well as in hospital.
Disadvantages
- Can rarely cause serious skin reactions and can lower blood cell counts, so it is not a routine everyday antibiotic.
- Not suitable for people with a sulfonamide (sulfa) allergy and generally avoided in early pregnancy.
- Can raise blood potassium and interacts with several common medicines, so it often needs blood monitoring.
Practical use
Good to know
Co-trimoxazole is kept for specific situations rather than used for every infection, mainly because of resistance and the small risk of serious reactions. The most important caution is allergy: if you have ever reacted to a sulfonamide (sulfa) medicine, you should not take it. It can occasionally cause serious skin reactions (such as widespread rash with blistering or peeling), so any new spreading rash, mouth ulcers, blisters or peeling skin should be treated as an emergency and the medicine stopped. It can also lower blood cell counts and push up the level of potassium in the blood, so blood tests are often done, especially with longer courses, in older people, or alongside other medicines that raise potassium. It interferes with folate, so it is generally avoided in pregnancy (particularly early on) unless a specialist judges the benefit outweighs the risk, and folic acid may be advised in some situations. Drink plenty of fluids while taking it, and tell your prescriber about all your other medicines, as it interacts with several of them.
Who should not take it / use with caution
- People with a known allergy to sulfonamide (sulfa) medicines or to trimethoprim should not take it.
- It is generally avoided in pregnancy — particularly early on, and also close to delivery — and in people with severe kidney or liver problems unless a specialist advises.
- Used with great caution in people with certain blood disorders, folate deficiency, a high blood potassium level, or the inherited condition G6PD deficiency (where it can trigger red-cell breakdown).
Monitoring
- Blood tests to check blood cell counts, kidney function and potassium, particularly with longer courses or in older people.
- Watching closely for any new or spreading rash, mouth ulcers or peeling skin as an early sign of a serious reaction.
- Reviewing other medicines for interactions, including warfarin and potassium-raising treatments.
Side effects
- Nausea, being sick, loss of appetite and a skin rash are among the more common effects.
- It can lower blood cell counts and raise blood potassium, which is why blood tests are often arranged.
- Rarely, it can cause a serious widespread skin reaction with blistering or peeling, which needs emergency care.
Key interactions
- Can increase the effect of warfarin, raising the risk of bleeding, so closer monitoring may be needed.
- Combined with medicines that raise potassium (such as some blood-pressure tablets and potassium supplements), it can push potassium too high.
- Can affect levels of methotrexate, some diabetes tablets and phenytoin, so these combinations need care.
Available as: Tablets and oral liquid; also given by infusion in hospital.
Answers
Co-trimoxazole: frequently asked questions
What is co-trimoxazole mainly used for?
In the UK it is used above all to treat and prevent PCP (Pneumocystis pneumonia) in people with a weakened immune system, and for some other specific infections rather than everyday ones.
Is it just an ordinary antibiotic?
It is a combination of two antibiotics kept for particular situations; it is not usually a first choice for common infections because of resistance and a small risk of serious reactions.
Can I take it if I am allergic to sulfa medicines?
No. If you have ever reacted to a sulfonamide (sulfa) medicine you should not take co-trimoxazole, so tell your prescriber about any such allergy.
Why might I need blood tests?
It can lower blood cell counts and raise potassium, so blood tests are often arranged, especially with longer courses, in older people, or alongside certain other medicines.
When should I seek urgent help?
Stop it and seek emergency advice if you develop a spreading rash, blisters, peeling skin, mouth ulcers or a sore throat with fever, as these can signal a serious reaction.
Authoritative sources
- BNF
- NICE CKS
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