Histamine antagonist

A histamine antagonist is an agent which serves to inhibit the release or action of histamine. Antihistamine can be used to describe any histamine antagonist, but it is usually reserved for the classical antihistamines that act upon the H1 histamine receptor.

H1-receptor antagonists
In common use, the term antihistamine refers only to H1 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. Clinically, H1 antagonists are used to treat allergic reactions.

Examples:
 * Diphenhydramine
 * Loratadine
 * Meclizine
 * Quetiapine

H2-receptor antagonists
H2 antagonists, like H1 antagonists, are also inverse agonists and not true antagonists. H2 histamine receptors are found principally in the parietal cells of the gastric mucosa. H2 antagonists are used to reduce the secretion of gastric acid, treating gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease.

Examples:
 * Cimetidine
 * Famotidine
 * Ranitidine

Experimental: H3- and H4-receptor antagonists
These are experimental agents and do not yet have a defined clinical use.

H3-receptor antagonists
Examples:
 * ABT-239
 * Cipralisant
 * Ciproxifan
 * Clobenpropit
 * Thioperamide

H4-receptor antagonists
Examples:
 * Thioperamide

Inhibitors of histamine release
These agents (mast cell stabilizers) appear to stabilize the mast cells to prevent degranulation and mediator release.

Examples:
 * Cromoglicate (cromolyn)
 * Nedocromil

Other agents with antihistaminergic activity
Many drugs, used for other indications, possess unwanted antihistaminergic activity. These include tricyclic antidepressants and antipsychotics.