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A drug may be classified by the chemical type of the active ingredient or by the way it is used to treat a particular condition. Each drug can be classified into one or more drug classes.

Sulphonamides (sulfa drugs) are one of a group of drugs derived from sulphanilamide that prevents the growth of bacteria.

Sulphonamides compete with p-aminobenzoic acid (PABA) for the enzyme dihydropteroate synthetase, which is important in the formation of folic acid that is required by the bacteria. Folic acid is required for the synthesis of precursors of DNA and RNA both in bacteria and in mammals. Mammals obtain their folic acid in their diet but bacteria need to synthesize it. Sulphonamides inhibit the growth of bacteria but do not kill them i.e. their action is bacteriostatic.

Many sulphonamides are rapidly excreted and very soluble in urine so they are used to treat infections of the urinary tract.

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