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Co-trimoxazole (abbreviated SXT, TMP-SMX, TMP-SMZ or TMP-sulfa) is a sulfonamide antibiotic...

Co-trimoxazole (abbreviated SXT, TMP-SMX, TMP-SMZ or TMP-sulfa) is a sulfonamide antibiotic combination of trimethoprim and sulfamethoxazole, in the ratio of 1 to 5, used in the treatment of a variety of bacterial infections. The name co-trimoxazole is the British Approved Name, and has been marketed worldwide under many trade names including Septra (GSK), Bactrim (Roche), and various generic preparations. Sources differ as to whether co-trimoxazole usually is bactericidal or bacteriostatic. Co-trimoxazole exhibits a theoretical, although perhaps not a clinical, synergistic antibacterial effect when compared to each of its components administered singly. This is because trimethoprim and sulfamethoxazole inhibit successive steps in the folate synthesis pathway (see diagram below). They did not exhibit synergistic effects, due to the requirement of a 1 in 5 ratio, which was observed in the laboratory. However, in clinical situations the ratio in the tissue was observed to be 1 in 20, resulting in no synergy. Sulfamethoxazole acts as a false-substrate inhibitor of dihydropteroate synthetase. Sulfonamides such as sulfamethoxazole are analogues of p-aminobenzoic acid (PABA) and are