noun a muscle that relaxes while another contracts.
noun a drug that neutralizes or counteracts the effects of another drug.
Examples
“2001: The calcium antagonists represent one of the top ten classes of prescription drugs in terms of commercial value, with worldwide sales of nearly $10 billion in 1999. — Leslie Iversen, Drugs: A Very Short Introduction (Oxford 2001, p. 41)”
“A flexor, which bends a part, is the antagonist of an extensor, which extends it.”