ANTAGONISM

in chemotherapy, any action by which one drug opposes, resists, or counteracts the effects of another. Antagonism may result from changes in the distribution, biotransformation, or excretion of the drugs or may occur because the two drugs act at the same specific cell component (receptor antagonism). A drug that combines with receptors and initiates action has both affinity ( i.e., the drug is localized at the receptor) and intrinsic activity (efficacy); such drugs are called agonists. Sulfonamides, for example, are agonists; they act by preventing the incorporation by certain bacteria of a substance (p-aminobenzoic acid, PABA) necessary for their growth. Substances closely related to PABA (e.g., procaine) have an action similar to it and counteract (i.e., antagonize) sulfonamide action. In some instances, drug antagonists are deliberately combined, as when digitalis, a stimulant, and quinidine, a depressant, are used to treat congestive heart failure (physiological antagonism). In other instances, antagonism is not advantageous, as when aspirin and probenecid are used to treat gout; either is effective alone, but the action of probenecid is prevented by aspirin when the two are used together.