Medical Definition of agonist
1: a muscle that on contracting is automatically checked and controlled by the opposing simultaneous contraction of another muscle—called also agonist muscle, prime mover; compare antagonist a, synergist 2
2: a chemical substance (as a drug) capable of combining with a receptor on a cell and initiating the same reaction or activity typically produced by the binding of an endogenous substance <binding of adrenergic agonists>—compare antagonist b
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