an•tag•o•nist(antag´ənist) n.
 1 a person who opposes or competes with another; adversary; opponent
 2 a muscle that counteracts or opposes the action of another muscle (called the agonist)
 3 a substance in the body, or a drug, that counteracts or blocks the effect of another substance or drug (called the agonist)
—SYN. opponent