n :  an agent that acts in physiological opposition <contact between a tooth and its in the opposing jaw> : as  a  :  a muscle that contracts with and limits the action of an agonist with which it is paired - called also antagonistic muscle  - compare AGONIST1 SYNERGIST2    b  :  a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate) : esp  :  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 AGONIST2   .
 
 
 
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