Co-trimoxazole (Septrin) is an approved antibiotic that consists of a mixture of two drugs, trimethoprim and sulphamethoxazole. Both drugs prevent bacteria from reproducing by preventing the production of folic acid (vitamin B₉).

The tablet form of co-trimoxazole is most commonly used by people with HIV as prophylaxis against Pneumocystis pneumonia (PCP). It also offers a high level of protection against toxoplasmosis and bacterial infections^[1]. Co-trimoxazole may also be used to treat Shigella, urinary tract infections, isosporiasis and bronchitis. Intravenous co-trimoxazole is used for treating PCP, Shigella, Salmonella and severe or complicated urinary tract infections. Co-trimoxazole is also available as a suspension for children.

In resource-poor countries, co-trimoxazole can reduce the incidence of a range of opportunistic infections including pneumonia and isosporiasis, even in patients with tuberculosis^[2][3]. It also has an anti-malarial effect^[4]. In 2000, the World Health Organization recommended that all HIV-positive patients with CD4 cell counts below 500 cells/mm³ in sub-Saharan Africa take co-trimoxazole to prevent infections. Following a halving of death rates in a large randomised study in an area with high levels of resistance to the drug, this recommendation was modified in November 2004 to include all HIV-infected or -exposed children, until immune restoration has occurred or HIV-negativity is demonstrated^[5]. Some experts believe that all infected adults should also use the drug^[6].

Co-trimoxazole can be used by HIV-positive pregnant women, since the risk of foetal abnormalities is low. In resource-limited settings, the benefits of co-trimoxazole in preventing other infections far exceeds the risk of birth defects^[7].

In July 1995, the United Kingdom Department of Health announced that co-trimoxazole’s licence was to be restricted since trimethoprim alone is just as effective, although the risk of severe side-effects seems to be similar for either treatment. However, co-trimoxazole remains the recommended drug for PCP and toxoplasmosis treatment and prophylaxis, as well as certain serious infections that are unresponsive to trimethoprim alone. In animal studies, trimethoprim alone is not effective against PCP.

Co-trimoxazole is excreted through the kidneys, so people who have kidney damage may have to take lower doses to avoid the risk of side-effects such as low blood sugar levels^[8].

The drug may decrease the effectiveness of oral contraceptives. The effectiveness of co-trimoxazole may also be reduced if it is taken alongside vitamin supplements containing folic acid.

Co-trimoxazole can cause a high level of side-effects, especially in the intravenous formulation. The most common reactions are rash, fever, nausea, low white blood cells and liver inflammation. Nausea may be reduced if the drug is taken with food. It should be taken with plenty of water.

Skin rashes due to co-trimoxazole are common among HIV-positive people. These should be taken seriously and treatment stopped or the dose reduced because some people can develop a potentially fatal allergy called Stevens-Johnson syndrome. People who develop severe allergic reactions to co-trimoxazole appear to be at increased risk of rapid disease progression, for reasons that remain unclear^[9]. Gradually introducing the drug to the body, through dose escalation reduces the chance of an allergic reaction. These desensitisation regimens are designed to enable people who are allergic to co-trimoxazole to continue to take the drug without problems. In some cases, steroids can be given to reduce the allergic responses that can still occur during desensitisation, and some people require two or three attempts at the procedure before it succeeds. Research shows that desensitisation leads to fewer opportunistic infections^[10][11].

It is generally accepted that most of the side-effects from co-trimoxazole are caused by the sulphamethoxazole component rather than the trimethoprim. Inherited differences make some people more susceptible to this metabolite, possibly explaining why side-effects are more common among Caucasians than people of other ethnic backgrounds^[12][13].

Resistance to co-trimoxazole may develop, particularly among people who have previously taken the drug for treatment or prevention of PCP, and those who have used pyrimethamine with sulfadoxine (Fansidar) to prevent or treat malaria. Resistance to co-trimoxazole increases a person’s risk of death from PCP. However, people with low-level resistance may still benefit from co-trimoxazole therapy^[14].