The two components act synergistically to produce a bactericidal or bacteriostatic action against a wide range of gram-positive and gram-negative organisms and some protozoa. It is also active against pneumocystis carinii. Sulphamethoxazole inhibits the enzyme dihydropteroate reductase, which catalyzes the reduction of folic acid to dihydrofolic acid whilst trimethoprim prevents the reduction of dihydrofolic acid to tetrahydrofolic acid by competitive inhibition of dihydrofolate reductase, thereby interfering with cell replication.
Both drugs are rapidly and almost completely absorbed from the bowel reaching peak plasma concentrations 1-4 hours after ingestion. Sulphamethoxazole is 65% and trimethoprim is 40% protein bound.