- - 1. Any structure or substance that opposes or counteracts the action of another structure or substance
- - 2. In pharmacology, a chemical that occupies a receptor site on the cell membrane but does not initiate the biologic reaction associated with occupation of the site by an agonist; in effect, an antagonist interferes with the formation of an agonist-receptor complex, the mechanism by which most pharmacologic effects are produced.
competitive a. A drug that interacts reversibly with the same set of receptors as the active drug to form a complex, but the complex does not elicit a biologic response and can be displaced from these receptor sites by increasing concentrations of the active drug.