Sulphonamides, amongst the first successful chemotherapeutic agents, now have their place in medicine mainly in combination with trimethoprim. Because of the risks of adverse drug reactions associated with their use, this is generally restricted to specific indications where other therapeutic agents have clearly inferior efficacy. Many sulphonamide compounds have been withdrawn from the market. Their individual names are standardised in the UK to begin with 'sulfa-'.

The enzyme dihydrofolic acid (DHF) synthase (see below) converts p-aminobenzoic acid (PABA) to DHF which is subsequently converted to tetrahydric folic acid (THF), purines and DNA. The sulphonamides are structurally similar to PABA, successfully compete with it for DHF synthase and thus ultimately impair DNA formation. Most bacteria do not use preformed folate, but humans derive DHF from dietary folate which protects their cells from the metabolic effect of sulphonamides. Trimethoprim acts at the subsequent step by inhibiting DHF reductase, which converts DHF to THF. The drug is relatively safe because bacterial DHF reductase is much more sensitive to trimethoprim than is the human form of the enzyme. Both sulphonamides and trimethoprim are bacteriostatic.

Pharmacokinetics. Sulphonamides for systemic use are absorbed rapidly from the gut. The principal metabolic path is acetylation and the capacity to acetylate is genetically determined in a bimodal form, i.e. there are slow and fast acetylators (see Pharmacogenetics) but the differences are of limited practical importance in therapy. The kidney is the principal route of excretion of drug and acetylate.

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