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sulphonamide (sulpha drug) (sul-fon-ă-myd) n. one of a group of drugs, derived from sulphanilamide (a red dye), that prevent the growth of bacteria. Sulphonamides are effective against a variety of infections and were formerly widely used, particularly in treating urinary-tract infections because many of them are rapidly excreted and very soluble in the urine. However, because of their toxic side-effects, the clinical use of sulphonamides has now declined. See co-trimoxazole, sulfadiazine, sulfasalazine.