Interactions among antibiotics and other drugs.

With the increasing use of multiple therapeutic agents, it has become increasingly evident that the pharmacologic action of a drug may be quantitatively altered in patients receiving other drugs. Antibiotics may interact with unrelated nonantibiotic drugs, and the result may be the enhanced or decreased activity of the antibiotic or other drug. Two antibiotics may be administered in combination in order to (1) delay emergence of resistant organisms; (2) treat mixed or undiagnosed infections; or (3) enhance the rate of bactericidal action. Whether a combination of antibiotics will be synergic, additive, indifferent, or antagonistic frequently is not predictable. In vitro tests are required to determine synergic combinations. Interactions and the mechanisms of these interactions involving antimicrobial agents in man are reviewed in this communication. Interactions Among Antimicrobial and Nonantimicrobial Drugs The magnitude of the problem of drug interactions is unknown, and it is probable that the clinician is unaware of