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Active ingredient

Sulfamethoxazole + Trimethoprim

Sulfamethoxazole with trimethoprim, often called co-trimoxazole (and known by brand names such as Bactrim), is a combination antibiotic that pairs two active ingredients in a fixed ratio. It is prescribed for certain bacterial infections, including some urinary tract infections and particular chest infections, and has an established role in preventing and treating a type of pneumonia (Pneumocystis pneumonia) in vulnerable people. It is effective only against susceptible bacteria, not viruses.

Reviewed by the Kam4eu Pharmacy Team on 21 June 2026 · Next review June 2027

Sulfamethoxazole + Trimethoprim medicines we deliver

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1 generic Sulfamethoxazole + Trimethoprim product — same molecule, different brands, formats and pack sizes.

How it works

The two ingredients act on the same bacterial pathway at different steps. Bacteria need to make folic acid (folate) to build DNA and multiply. Sulfamethoxazole blocks an early step in folate production, while trimethoprim blocks a later step. By interrupting the pathway at two points, the combination has a stronger effect on susceptible bacteria than either drug alone. Human cells obtain folate from the diet and do not rely on this pathway, allowing the medicine to act more selectively.

How it is taken

Take co-trimoxazole exactly as prescribed and read the patient information leaflet carefully. It comes as tablets and an oral liquid. It is generally recommended to take it with a meal or snack and to drink plenty of fluids during treatment to help protect the kidneys. Take doses at evenly spaced times and complete the full course even if you feel better, to help fully clear the infection and limit antibiotic resistance.

Side effects

Common (usually mild and temporary):

  • Nausea or vomiting
  • Diarrhoea
  • Loss of appetite
  • Skin rash or itching
  • Headache
  • Sore or inflamed mouth or tongue

Stop and seek medical help if you experience:

  • Signs of a severe allergic reaction such as swelling of the face, lips, tongue or throat, breathing difficulty, or collapse — seek emergency care
  • Severe, blistering or peeling skin rashes, or sores in the mouth, eyes or genitals — stop and get urgent help
  • Signs of a blood disorder such as unexplained bruising or bleeding, persistent sore throat, high fever or extreme tiredness
  • Yellowing of the skin or eyes, severe stomach pain, or other signs of liver problems
  • Severe or persistent watery or bloody diarrhoea during or after treatment

Safety & who should not take it

  • Tell your doctor if you are allergic to sulfonamide (sulfa) medicines, trimethoprim or other antibiotics, as serious allergic reactions can occur.
  • Inform your doctor of any kidney or liver problems, blood disorders, folate deficiency, asthma, or a deficiency of the enzyme G6PD before starting.
  • Generally avoided in pregnancy and in very young infants; tell your doctor if you are pregnant, planning a pregnancy or breastfeeding.
  • List all other medicines, as co-trimoxazole interacts with several drugs, including some blood thinners, diabetes medicines and water tablets (diuretics).
  • Drink plenty of fluids during treatment and tell your doctor if you produce much less urine than usual.
  • Use only for the infection it was prescribed for, and never share it, to help limit antibiotic resistance.
  • Return any unused or out-of-date medicine to a pharmacy for safe disposal.

Storage

Store in the original packaging below 25°C, protected from light and moisture, out of the sight and reach of children. Follow any specific shelf-life instructions for the oral liquid. Do not use after the expiry date.

Frequently asked questions

Why is co-trimoxazole two medicines in one?+

Combining sulfamethoxazole and trimethoprim blocks the same bacterial folate pathway at two separate steps, giving a stronger effect against susceptible bacteria than either ingredient alone.

Why should I drink plenty of water with it?+

Drinking adequate fluids helps keep urine flowing and supports the kidneys during treatment. Your doctor or pharmacist can advise how much is appropriate for you.

Can I stop taking it once I feel better?+

No. Finish the full course as prescribed, even if symptoms improve, to help clear the infection completely and reduce the risk of antibiotic resistance.

Important: This information is general and not a substitute for medical advice. Always read the patient information leaflet supplied with your medicine and consult a doctor or pharmacist before use — especially if you have existing health conditions or take other medication.