BMA Concise Guide to Medicine & Drugs


Brand name Septrin

Used in the following combined preparation (Co-trimoxazole is a combination of two drugs)


Drug group Antibacterial drug

Overdose danger rating Medium

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Co-trimoxazole is a mixture of two antibacterial drugs: trimethoprim and sulfamethoxazole. It is prescribed for serious respiratory and urinary tract infections only when they cannot be treated with other drugs. Co-trimoxazole is also used to treat pneumocystis pneumonia, toxoplasmosis, and the bacterial infection nocardiasis. The drug may also be used for otitis media in children if no safer drug is suitable. Although co-trimoxazole was widely prescribed in the past, its use has greatly declined in recent years with the introduction of new, more effective, and safer drugs.

Rare but serious adverse effects of co-trimoxazole may occur and these include skin rashes, blood disorders, and liver or kidney damage.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, liquid, injection.

Frequency and timing of doses Normally 2 x daily, preferably with food.

Adult dosage range Usually 4 tablets daily (each standard tablet is 480mg). Higher doses may be used for the treatment of pneumocystis pneumonia, toxoplasmosis, and nocardiasis.

Onset of effect 1–4 hours.

Duration of action 24 hours.

Diet advice Drink plenty of fluids, particularly in warm weather.

Storage Keep in original container at room temperature out of the reach of children. Protect from light.

Missed dose Take as soon as you remember. If your normal dose is 480mg, double this; if it is more than 480mg, take one dose only.

Stopping the drug Take the full course. Even if you feel better, the original infection may still be present and symptoms may recur if treatment is stopped too soon.

Exceeding the dose An occasional unintentional extra dose is unlikely to be a cause for concern. Large overdoses may cause nausea, vomiting, dizziness, and confusion. Notify your doctor.


The most common adverse effects of co-trimoxazole are nausea, rash, and itching. If either of the last two occur, you should stop taking the drug and consult your doctor without delay. Diarrhoea and headache are also relatively common; consult your doctor if they are severe. More rarely, mouth ulcers, sore tongue, or jaundice may occur; if so, stop taking the drug and consult your doctor promptly.


Warfarin Co-trimoxazole may increase its anticoagulant effect; the dose of warfarin may have to be reduced. Blood-clotting status may have to be checked.

Ciclosporin Taking ciclosporin with co-trimoxazole can impair kidney function.

Phenytoin Co-trimoxazole may cause a build-up of phenytoin in the body; the dose of phenytoin may have to be reduced.

Amiodarone Co-trimoxazole may increase the risk of irregular heart beats when given with amiodarone.

Methotrexate Co-trimoxazole may increase the blood level of methotrexate and regular blood tests may be necessary.


Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have a blood disorder.

· You have asthma.

· You have glucose-6-phosphate dehydrogenase (G6PD) deficiency.

· You are allergic to sulphonamide drugs.

· You suffer from porphyria.

· You are taking other medicines.

Pregnancy Not prescribed. May cause defects in the baby.

Breast-feeding The drug passes into the breast milk, but at normal levels adverse effects on the baby are unlikely. Discuss with your doctor.

Infants and children Not recommended in infants under 6 weeks old. Reduced dose necessary in older children.

Over 60 Side effects are more likely. Used only when necessary.

Driving and hazardous work No known problems.

Alcohol No known problems.


Long-term use of this drug may lead to folic acid deficiency, which can cause anaemia. Folic acid supplements may be needed.

Monitoring Regular blood tests are recommended.