noun

• (biochemistry) A chemical that binds to a receptor but does not produce a physiological response, blocking the action of other agonist chemicals, such as endogenous chemical messengers.
  "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)"
• (authorship) The main character or force opposing the protagonist in a literary work or drama.
  "So God forbid I'm seen just as an average human being / I mean, imagine if antagonists lacked any evil scheme"
• (anatomy) A muscle that acts in opposition to another.
  "A flexor, which bends a part, is the antagonist of an extensor, which extends it."