sulfamethoxazole n a sulfonamide (trade name Gantanol) used to treat infections (especially infections of the urinary tract) Usage(s) British doctors have been using co-trimoxazole, a combination of the antibiotics trimethoprim and sulfamethoxazole, since 1968. British doctors have been using co-trimoxazole, a combination of the antibiotics trimethoprim and sulfamethoxazole, since 1968. Synonym(s) Gantanol