An antagonist is a ligand that binds a receptor but does not elicit the response associated with receptor activation. Instead, occupancy of the binding site prevents agonists from producing their effect. Antagonists therefore have measurable affinity but, by definition, zero intrinsic efficacy at the receptor of interest.
Antagonism can be competitive — reversible binding at the orthosteric site, shifting the agonist dose-response curve rightward without lowering its maximum — or non-competitive, in which case both the position and the maximum of the agonist curve are affected. Antagonist potency is characterised by IC50 or, more rigorously, by Kᵢ or pA₂.
Antagonists are essential pharmacological tools for confirming that a proposed receptor mediates the effect of an agonist.