Sulfamethoxazole is a bacteriostatic sulfonamide antibiotic that interferes with folic acid synthesis in susceptible bacteria. It is generally given in combination with [trimethoprim], which inhibits a sequential step in bacterial folic acid synthesis – these agents work synergistically to block two consecutive steps in the biosynthesis of nucleic acids and proteins which are necessary for bacterial growth and division, and using them in conjunction helps to slow the development of bacterial resistance. In this combination, sulfamethoxazole is useful for the treatment of a variety of bacterial infections, including those of the urinary, respiratory, and gastrointestinal tracts.