Trimethoprim–sulfamethoxazole (TMP-SMX) is a fixed combination of two antimicrobial agents — a diaminopyrimidine (trimethoprim) and a sulfonamide (sulfamethoxazole) — purposely chosen to provide sequential and synergistic inhibition of bacterial folate metabolism. Although a parenteral formulation of this combination is currently under evaluation, TMP-SMX is generally available only for oral use. It is marketed in a fixed drug ratio of one part TMP to five parts SMX (single-strength tablets contain 80 mg of TMP and 400 mg of SMX, double-strength tablets contain 160 mg of TMP and 800 mg of SMX, and an oral suspension contains 40 mg of TMP and . . .