Trimethoprim antibiotic pill on its blister pack. Trimethoprim is primarily used to treat infections of the urinary tract. It works by inhibiting the bacterial enzyme dihydrofolate reductase, which is needed for bacterial synthesis of the essential chemical folic acid, needed for DNA replication. Trimethoprim is often taken in conjunction with sulfamethoxazole, another antibiotic, which targets another part of the bacterial folic acid synthesis pathway, increasing the effect of the treatment. This combination is known as co- trimoxazole.