Trimethoprim is a synthetic antibiotic that inhibits dihydrofolate reductase (DHFR) which is necessary for the synthesis of purines, amino acids, and thymidylic acid, with prominent selectivity for bacterial DHFR over mammalian DHFR (IC50s = 5 and 30,000 nM, respectively). Trimethoprim is used mainly in the prophylaxis and treatment of urinary tract infections.