Antihistamines

A H₁ antihistamine is a histamine antagonist which serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions, through action at the H1 receptor. Only 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-receptor antagonists, also known as H₁-antihistamines. It has been discovered that these H₁-antihistamines are actually inverse agonists at the histamine H₁-receptor, rather than antagonists per se.