Bactrim is an antibiotic drug formed by combining the sulfonamide sulfamethoxazole (5 parts) with trimethoprim (1 part). This combination (SMX-TMP) has in vivo activity against both Gram-positive and Gram-negative aerobic organisms. It has minimal activity against anaerobic organisms. Sulfonamides are structural analogs of para-aminobenzoic acid (PABA) and competitively inhibit the bacterial enzyme dihydropteroate synthetase that is responsible for incorporation of PABA into dihydrofolic acid, resulting in a decrease in the amount of metabolically active tetrahydrofolic acid.

Trimethoprim is a bacteriostatic lipophilic weak base structurally related to pyrimethamine. It binds and reversibly inhibits the enzyme dihydrofolate reductase and, like sulfonamide, selectively blocks the conversion of dihydrofolic acid to tetrahydrofolic acid, thus leading to depletion of folate, which results in interference of bacterial nucleic acid production. The individual effects of each component drug are magnified in combination and together may provide bactericidal effects.

SMX-TMP is rapidly absorbed after oral administration, with the bioavailability estimated at 90% to 100%. Sulfamethoxazole is primarily metabolized in the liver by acetylation to inactive metabolites that retain the toxicity of the parent compound. Trimethoprim is also metabolized in the liver, with 10% to 20% metabolized to inactive metabolites by o-demethylation ring N-oxidation and alp-hahydroxylation. Sulfamethoxazole is excreted renally by glomerular filtration with some tubular secretion. Excretion is increased in alkaline urine. Trimethoprim is 40% to 60% excreted within 24 hours primarily by tubular secretion. Excretion is increased in acid urine and decreased in alkaline urine.