AskDefine | Define antihistamine

Dictionary Definition

antihistamine n : a medicine used to treat allergies and hypersensitive reactions and colds; works by counteracting the effects of histamine on a receptor site

User Contributed Dictionary




  1. A drug or substance that counteracts the effects of a histamine. Commonly used to alleviate the symptoms of hay fever and other allergies.


Extensive Definition

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.
Many people suffer from allergies. These are caused by an excessive response of the body to allergens, materials that are sometimes omnipresent, such as the pollen released by grasses and trees. An allergic reaction indicates an excessive release, by the body, of histamines. The antihistamines described here inhibit the release of these histamines, thus relieving the allergic reactions and symptoms.

Clinical: H1- and H2-receptor antagonists

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.

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.

Experimental: H3- and H4-receptor antagonists

These are experimental agents and do not yet have a defined clinical use.

H3-receptor antagonists

H4-receptor antagonists



Inhibitors of histamine release

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

Other agents with antihistaminergic activity

Many drugs, used for other indications, possess unwanted antihistaminergic activity. These include tricyclic antidepressants and antipsychotics.
Large doses of vitamin C are known to alleviate shock by inhibiting deaminizing proteins which release histamine.


External links

antihistamine in Korean: 항히스타민제
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