Co-trimoxazole (trimethoprim with sulfamethoxazole, Septrin): uses and warnings

Co-trimoxazole is a fixed combination of sulfamethoxazole, a sulphonamide antibiotic, and trimethoprim. The patient leaflet explains that like all antibiotics it only works against some types of bacteria, so it is suitable only for some infections. In the United States, branded forms are sold as Bactrim or Septra; in the UK the usual names are co-trimoxazole and Septrin.

Trimethoprim is also available on its own, and the NHS says it is used to treat and prevent urinary tract infections such as cystitis, and occasionally other infections. Co-trimoxazole adds sulfamethoxazole, a sulphonamide, which brings the allergy and skin-reaction warnings described below.

The forte tablet leaflet says the tablets are indicated in adolescents over 12 and adults, and that consideration should be given to official guidance on the appropriate use of antibacterial agents.

The leaflet lists what it can be used to treat or prevent: lung infections caused by Pneumocystis jirovecii (pneumonia or PJP) and infections caused by Toxoplasma (toxoplasmosis). It can be used to treat urinary bladder or urinary tract infections, respiratory tract infections such as bronchitis, ear infections such as otitis media, and nocardiosis, an infection which can affect the lungs, skin and brain.

The leaflet says it can be used to prevent as well as treat PJP and toxoplasmosis. If you are given it as a preventive medicine, ask your team why you need it, how long you should continue and which blood tests are needed.

The leaflet says the standard dose for acute infections in adults is one forte tablet in the morning and one in the evening for at least five days, finishing the course, with different doses and durations for urinary tract infections, PJP, toxoplasmosis and nocardiosis. People with kidney problems may need a lower dose and blood tests, and long-term use may require folic acid.

The leaflet states that potentially life-threatening skin rashes, Stevens-Johnson syndrome, toxic epidermal necrolysis and drug reaction with eosinophilia and systemic symptoms (DRESS) have been reported. They appear initially as reddish target-like spots or circular patches, often with central blisters on the trunk, and additional signs include ulcers in the mouth, throat, nose and genitals and red, swollen eyes, often with flu-like symptoms. At the start of treatment, a generalised skin redness with pustules and fever may suggest acute generalised exanthematous pustulosis (AGEP).

The highest risk is within the first weeks of treatment. If you develop a rash or these skin symptoms the leaflet says to stop taking co-trimoxazole, seek urgent advice from a doctor and tell them you are taking it. If you have had one of these reactions you must not be restarted on it at any time.

The NHS describes Stevens-Johnson syndrome as a rare but serious skin reaction, usually caused by certain medicines, which needs to be treated immediately in hospital. It names sulfamethoxazole and other sulfa antibiotics among the medicines commonly linked to it, and says to call 999 or go to A&E for a rash spreading across the body, a rash that is painful or blistered or peeling, wheezing, tightness in the chest or throat, or a swollen or sore mouth, face, lips or eyes. The NHS stresses that the condition is rare and the risk is low even when taking a medicine known to cause it.

The leaflet says not to take it if you have severe liver or kidney problems, have had thrombocytopenia (a problem causing bruising or bleeding), have porphyria, or are an infant during the first 6 weeks of life. It asks you to talk to your doctor if you have kidney disease, folic acid deficiency, G6PD deficiency, phenylketonuria, are elderly, underweight or malnourished, or have a lot of potassium in your blood.

Hyperkalaemia, high potassium, is very common with co-trimoxazole. Symptoms of severe hyperkalaemia can include muscle cramps, irregular heart rhythm, diarrhoea, nausea, dizziness or headache. The leaflet lists diuretics such as spironolactone, steroids such as prednisolone, digoxin and ACE inhibitors among the medicines that can add to this, along with potassium supplements and potassium-rich food.

Other medicines to mention include warfarin, phenytoin, sulphonylureas and repaglinide for diabetes, methotrexate, azathioprine, ciclosporin, digoxin, rifampicin, pyrimethamine, zidovudine, lamivudine and contraceptives. Blood disorders are another concern: unexplained fever, frequent infections, bruising or bleeding should be reported.

The leaflet advises taking it with some food or drink to reduce sickness and diarrhoea, and drinking plenty of fluid. It also asks you to tell your doctor about an unexpected worsening of cough and shortness of breath.

The leaflet says the medicine should not be taken during the first three months of pregnancy. Treatment with trimethoprim in the first three months may increase the risk of miscarriage, and children born to mothers treated in the first trimester may have an increased risk of birth defects, in particular neural tube defects, oral clefts and heart defects. If you are pregnant, planning pregnancy or breastfeeding, ask your doctor or pharmacist for advice before taking it.

Co-trimoxazole is prescribed on the NHS in primary care and hospitals, by GPs and hospital specialists. The monitoring points above, blood tests, potassium and kidney function, apply in either sector. If you are taking it long term, ask who is responsible for your blood tests.