H₁ Antagonist

H₁ Antagonist

An H₁ antagonist is a histamine antagonist of the H₁ receptor that serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.

In common use, the term "antihistamine" refers only to H₁ antagonists, also known as H₁-receptor antagonists and H₁-antihistamines. It has been discovered that these H₁-antihistamines are actually inverse agonists at the histamine H₁-receptor rather than antagonists per se.