A histamine antagonist is an agent which serves to inhibit the release or action of histamine.
Although the term "antihistamine" could technically be used for any histamine antagonist, in practice the term is reserved for the classical antihistamines which act upon the H1 histamine receptor.
Clinical: H1- and H2-receptor antagonists
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. 
Clinically-relevant histamine H2-receptors are found principally in the parietal cells of the gastric mucosa. H2-receptor "antagonists" are also inverse agonists, rather than true antagonists; and are used to reduce the secretion of gastric acid. Examples include cimetidine, ranitidine, and famotidine.
Experimental: H3- and H4-receptor antagonists
These are experimental agents and do not yet have a defined clinical use.
- Similar in structure to burimamide
- Imidazole ring
- Polar moiety on the side chain - thiourea, isothiourea and guanidine
- other substituents are highly variable
Inhibitors of histamine release
These agents (mast cell stabilizers) appear to stabilise the mast cells to prevent degranulation and mediator release.
- Cromoglicate (cromolyn)
Other agents with antihistaminergic activity
Many drugs, used for other indications, possess unwanted antihistaminergic activity. These include tricyclic antidepressants, antipsychotics, etc.
- ↑ Leurs R, Church MK, Taglialatela M (2002). "H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects". Clin Exp Allergy. 32 (4): 489–98. PMID 11972592.
- Histamine+antagonist at the US National Library of Medicine Medical Subject Headings (MeSH)
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