Antibacterial drugs, such as quinolones, macrolides, rifampin, isoniazid, and trimethoprim-sulfamethoxazole, can interact with other drugs in a wide variety of clinically significant ways. They are frequently administered with other prescription and nonprescription medications. Antibacterial agents may interact by causing a change in the pharmacokinetics or pharmacodynamics of a second drug. In other cases, the antimicrobial may be affected by the action of another drug. Interactions involving antimicrobials often result from alterations in the absorption of the antimicrobial from the gastrointestinal tract or changes in the hepatic metabolism or renal elimination of the drugs concurrently administered. While certain classes of antibacterial drugs are known to interact with many other drugs, the interaction potential of most classes of antimicrobials is not uniform among members of the class. This diversity in interaction potential provides the clinician with an opportunity to avoid potential interactions by means of appropriate drug selection. An understanding of the common, clinically significant drug interactions involving antibacterial agents will enable the physician to avoid unnecessary adverse drug reactions.