Merriam-Webster Collegiate Dictionary - antagonist

 noun  Date: 1575  1. one that contends with or opposes another ; adversary, opponent  2. an agent of physiological antagonism: as  a. a muscle that contracts with and limits the action of an agonist with which it is paired — called also ~ic muscle  b. a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate); especially one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor — compare agonist 2b