antagonist

ot noun

1. A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals. 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)
2. A muscle that acts in opposition to another. A flexor, which bends a part, is the antagonist of an extensor, which extends it.
3. An opponent or enemy.
4. One who antagonizes or stirs.