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Mechanism of action

Sulphonamides act by inhibiting DNA synthesis. They interfere with the synthesis of folic acid, which is required by bacteria for making the subunits of DNA. Trimethoprim also acts by inhibiting folate synthesis, and the two agents in cotrimoxaole thus act synergistically (ie they enhance each other’s effect).

Route of administration

Cotrimoxazole can be given orally or intravenously. Some other sulphur compounds are available as topical antibiotics or antiseptics. As with the quinolones, the oral absorption of cotrimoxazole is very good.

Spectrum of action

Cotrimoxazole is active against a wide range of organisms, both Gram positive and Gram negative. Unfortunately, bacterial resistance to this agent is very common, and its use is thus restricted to certain specific clinical situations, or else to infections where the organism has been shown to be susceptible to cotrimoxazole. If an organism is susceptible, cotrimoxazole is a most effective and appropriate antibiotic.

Some of the specific indications for cotrimoxazole include:

• Pneumocystis carinii pneumonia (although this is not a bacterial infection – P. carinii is believed to be a fungus)
• Infections with Nocardia (this organism can cause chronic lung infections, brain abscesses and skin infections among others)

Other infections that can be treated with cotrimoxazole are:

• Urinary tract infections
• Respiratory tract infections
• Salmonella and Shigella infections

Side effects

Gastro-intestinal side effects are probably the commonest. Skin rashes are also relatively common with sulphur drugs. Sulphur drugs have been reported to cause problems in the bone marrow. However, this is very rare, and is probably more likely to occur in patients with pre-existing abnormalities of bone marrow, such as patients with leukaemia, or who have had bone marrow transplants.

Allergic reactions can also occur, and as with penicillin, can range from skin rashes to severe anaphylaxis.

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