Antagonist

A compound (usually synthetic) that inhibits the action of its natural counterpart by binding to a receptor or protein. There are three classes of antagonist, chemical, physiological and pharmacological.

Pharmacological antagonists can be further characterised as:

• Competitive which binds to the binding site of a receptor and can be overcome by increasing the concentration of an agonist (competitor).
• Irreversible competitive which forms a covalent bond with the receptor.
• Non-competitive antagonists which blocks the signal transduction event of the receptor.

A pharmacalogical example of an antagonist is Tamoxifen, a drug which acts on estrogen receptor. See also agonist.