<unk> s, also called<unk>  blockers, are a class of medications that block the action of histamine at the<unk>  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<unk> -antihistamines. Virtually all<unk> -antihistamines function as inverse agonists at the histamine<unk> -receptor, as opposed to neutral antagonists, as was previously believed.
H1 antagonist
GABA analogue
Muscarinic antagonist
Alpha-adrenergic agonist
Opioid antagonist
Beta-adrenergic agonist
5-HT3 antagonist
Beta2-adrenergic agonist