antagonist

  1. Substance that reverses or reduces the effect induced by an agonist.
  2. Substance that attaches to and blocks cell receptors that normally bind naturally occurring substances.
See also: agonist
Source:
PAC, 1993, 65, 2003 (Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)) on page 2014