Sulfamethoxazole inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid (PABA). Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the required enzyme, dihydrofolate reductase. Thus, sulfamethoxazole and trimethoprim blocks two consecutive steps in the biosynthesis of nucleic acids and proteins essential to many bacteria.

Urinary Tract Infections, Acute Otitis Media, Acute Exacerbations of Chronic Bronchitis in Adults, Shigellosis,  Pneumocystis Jiroveci Pneumonia, Traveler’s Diarrhea in Adults

Adults: The usual adult dosage in the treatment of urinary tract infections is 1 sulfamethoxazole and trimethoprim DS (double strength) tablet, USP every 12 hours for 10 to 14 days. An identical daily dosage is used for 5 days in the treatment of shigellosis.

Children: The recommended dose for children with urinary tract infections or acute otitis media is 40 mg/kg sulfamethoxazole, USP and 8 mg/kg trimethoprim, USP per 24 hours, given in two divided doses every 12 hours for 10 days. An identical daily dosage is used for 5 days in the treatment of shigellosis. The following table is a guideline for the attainment of this dosage:

The most common adverse effects are gastrointestinal disturbances (nausea, vomiting, anorexia) and allergic skin reactions (such as rash and urticaria).

Sulfamethoxazole and trimethoprim is contraindicated in patients with a known hypersensitivity to trimethoprim, USP or sulfonamides, in patients with a history of drug-induced immune thrombocytopenia with use of trimethoprim, USP and/or sulfonamides, and in patients with documented megaloblastic anemia due to folate deficiency.

Hypersensitivity and other fatal reactions, cough, shortness of breath and pulmonary infiltrates are hypersensitivity reactions of the respiratory tract that have been reported in association with sulfonamide treatment. Development of drug resistant bacteria.