H₁ antagonist

H₁ antagonists, also called H₁ blockers, are a class of medications that block the action of histamine at the H₁ receptor, helping to relieve 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-antihistamines. Virtually all H₁-antihistamines function as inverse agonists at the histamine H₁-receptor, as opposed to neutral antagonists, as was previously believed.