Trimethoprim is an inhibitor of the enzyme dihydrofolate reductase in the pathway of folic acid synthesis.
In combination with a sulphonamide, trimethoprim acts synergistically. Although active against the human dihydrofolate reductase enzyme, the bacterial enzyme is much more sensitive to trimethoprim.
Trimethoprim is active against many Enterobacteriaceae. The drug is excreted unchanged in urine.
Trimethoprim-sulfamethoxazole (co-trimoxazole) is drug of choice for the treatment of Pneumocystis carinii.
Trimethoprim is a bacteriostatic antimicrobial agent.
Extra info: Trimethoprim alone only inhibits bacterial growth, but in combination with sulfonamide it is bactericidal.