What is trimethoprim and how does it work?

Trimethoprim is a synthetic (man-made) antibiotic that interferes with the production of tetrahydrofolic acid, a chemical that is necessary in order for bacteria and human cells to produce proteins. Trimethoprim inhibits production of tetrahydrofolic acid by inhibiting the enzyme responsible for making tetrahydrofolic acid from dihydrofolic acid. Trimethoprim inhibits the bacterial enzyme more than the human enzyme. Therefore, Primsol has less effect on the production of tetrahydrofolic acid by humans. Because of the frequent development of resistance to Primsol, it is more effective when combined with another antibiotic, sulfamethoxazole (Azo-Gantanol), and is rarely used alone.