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Septrin (cotrimoxazole)

Michael Carter, Greta Hughson
Published: 06 June 2012

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


HIV-positive people are at increased risk of getting PCP if their CD4 cell count falls below 200. HIV treatment is recommended if your CD4 count falls below 350, but 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 AIDS. 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 dose for prophylaxis is one double strength (960mg) tablet every day. An alternative dose is one tablet three times a week.


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 an intravenous form is used instead of tablets. It may take five to seven days before you start to feel better, and the treatment usually needs to be continued for two to three weeks.

After the PCP has been successfully treated, it is important to take PCP prophylaxis to reduce the risk of it recurring. This is called maintenance therapy or secondary prophylaxis.


Some people are allergic to Septrin and develop reactions such as an itchy red rash, sometimes with fever. In affected people, this usually occurs during the second week of taking the drug.

In rare cases these reactions are extremely serious, so they should be reported to your doctor at once.

Anaemia (shortage of red blood cells) is the most common 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 AZT (Retrovir) or ganciclovir. Some people taking Septrin also experience liver problems. Blood tests can monitor both your blood cell levels and your liver function. People with kidney damage may need to take lower doses.

Other possible effects of Septrin include reduced levels of folate in the body, which can be corrected by a prescription of folinic acid, and a decrease in the effectiveness of oral contraceptives.


Because Septrin is the best form of PCP prophylaxis, doctors will often try to overcome allergic reactions you experience. 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 except under very close supervision in hospital.


If you are unable to tolerate Septrin, there are several alternatives that provide some protection against PCP, including aerosolised pentamidine, dapsone plus trimethoprim and atovaquone.

Contact NAM to find out more about the scientific research and information used to produce this factsheet.

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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