Factsheet Septrin (cotrimoxazole)

Selina Corkery, Published December 2017

Key points

  • This antibiotic is needed by people who have a low CD4 count.
  • It prevents infection with pneumonia and toxoplasmosis.
  • Higher doses can also be used to treat pneumonia.
  • Some people have allergic reactions to Septrin.

Septrin is the brand name for a combination of antibiotics called cotrimoxazole. Cotrimoxazole is the main drug used to treat and to prevent a type of pneumonia called PCP (Pneumocystis carinii pneumonia, now called Pneumocystis jiroveci pneumonia).


People living with HIV are at increased risk of getting PCP if their CD4 cell count falls below 200. In the UK, it is recommended you start HIV treatment as soon as you are diagnosed, whatever your CD4 cell count. However, if your CD4 count is below 200, it is also recommended that you take medication to reduce the risk of PCP until your CD4 cell count increases. This kind of treatment is called prophylaxis, which means it is to prevent infection, rather than to treat it.

PCP is a potentially fatal illness which used to be the most common cause of death for people with HIV. However, it is now less common as a result of effective HIV treatment, the use of PCP prophylaxis, and better treatments for people who do develop PCP.

Septrin is the most effective drug at preventing PCP, especially for people with CD4 counts below 100. It also reduces the risk of toxoplasmosis, an infection that can affect the brain.

The most common doses for prophylaxis are one double-strength (960mg) tablet or one single-strength tablet (480mg) every day. They have much the same effect, but the lower dose causes fewer side-effects. An alternative dose is one tablet three times a week, but this may not be as effective.


Septrin is also the first choice for treating people who do develop PCP. The dose used for treating PCP is higher than that used for preventing PCP. In some cases, if the PCP is more severe, you may be given Septrin intravenously. The recommended treatment lasts 21 days. It may be possible to switch to tablet form of Septrin if you respond well to the treatment. It may take five to seven days before you start to feel better. You may also be given a steroid, prednisone, to relieve your symptoms.

After the PCP has been successfully treated, you will need to take PCP prophylaxis to reduce the risk of it recurring. This is called maintenance therapy or secondary prophylaxis. You should be able to stop the maintenance therapy once your CD4 cell count has risen and your HIV viral load has been undetectable for some time.


A very small number of people develop a very serious reaction called Stevens Johnson syndrome (SJS) as a side-effect of treatment with Septrin. Skin reactions such as SJS may cause severe rash, crusting or ulcers of the mouth or genitals, burning skin and large layers of skin to flake off. See your HIV clinic immediately (or A&E if out of hours) if you develop a rash together with any of these symptoms:

  • fever
  • feeling generally unwell or extremely tired
  • muscle or joint ache
  • blistering of the skin
  • mouth ulcers
  • swelling of the eyes, lips, mouth or face
  • breathing difficulties
  • yellowing of the skin or eyes
  • dark urine
  • pale stools
  • pain, aching or sensitivity on the right-hand side of the body, below the ribs.

In rare cases, these reactions can be life-threatening, so you should seek medical advice at once.

Anaemia (shortage of red blood cells) is a possible side-effect experienced at the higher doses used for treating PCP. It can also affect people using Septrin for prophylaxis. Some people also experience a shortage of white blood cells. These side-effects are more common if you are also taking certain medications such as zidovudine (AZT, Retrovir) or ganciclovir. Some people taking Septrin also experience liver problems or increased potassium levels. Blood tests can monitor both your blood cell levels and your liver function. If you are on longer-term treatment with Septrin, you will have regular blood tests to monitor the levels of different blood cells, and other elements, in your blood.

People with kidney damage may need to take lower doses.

Some common side-effects of Septrin include headache, nausea, diarrhoea, and skin rash.

Other possible effects of Septrin include reduced levels of folate (a type of vitamin B) in the body. This can be corrected by a prescription of folinic acid.


Because Septrin is the best form of PCP prophylaxis, doctors will often try to overcome allergic reactions you experience. If your original allergic reaction was not serious, they may suggest trying again a few weeks later, as it is not always clear whether an allergy is due to Septrin or to other drugs. Alternatively you may be able to overcome the allergy by starting again at a very small dose and gradually increasing it to the normal level. This is called desensitisation.

"Septrin is the most effective drug at preventing PCP, especially for people with CD4 counts below 100."

Many people who have an initial reaction to Septrin can be desensitised in this way. However, you should not try this yourself. It should only be done under the supervision of a doctor. If your allergic reaction is severe, it is unwise to take the drug again.


If you are unable to tolerate Septrin, there are several alternatives that provide some protection against PCP, including clindamycin with primaquine, pentamidine, or dapsone plus trimethoprim. It is not recommended to use Septrin if you are pregnant.

This factsheet is due for review in December 2020

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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.