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antihistamine (ănˌtĭhĭsˈtəmēn), any one of a group of compounds having various chemical structures and characterized by the ability to antagonize the effects of histamine. Their principal use in medicine is in the control of allergies such as hay fever and hives. Some antihistamines are also useful as sedatives and for the prevention of motion sickness; others, such as fexofenadine (Allegra) and loratadine (Claritin) are nonsedating.
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A type of drug that inhibits the combination of histamine with histamine receptors. These drugs are termed either H-1 or H-2 receptor antagonists depending on which type of histamine receptor is involved. H-1 receptor antagonists are used largely for treating allergies, and H-2 receptor antagonists are used to treat peptic ulcer disease and related conditions. See Histamine

The primary therapeutic use of H-1 receptor antagonists is to antagonize the effects of histamine released from cells by antigen-antibody reactions; they can thus inhibit histamine-induced effects, such as bronchoconstriction, skin reactions, for example, wheals and itching, and nasal inflammation. These drugs, therefore, are quite effective in reducing allergy signs and symptoms, especially if they are administered before contact with the relevant antigen; however they are not effective in treating asthma. Their effects vary widely, both among the drugs and from individual to individual; in young children excitement may be seen. Another common set of effects caused by many of these drugs, including dry mouth, blurred vision, and urinary retention, can be ascribed to their anticholinergic actions. H-1 receptor antagonists have low toxicity. The chief adverse effect is sedation. Overdoses of H-1 receptor antagonists may be associated with excitement or depression, and although there is no pharmacologic antidote for these drugs, good supportive care should be adequate in managing cases of poisoning. See Allergy, Antigen-antibody reaction

H-2 receptor antagonists are much newer. Histamine stimulates gastric acid secretion by combining with H-2 receptors. By preventing this combination, H-2 antagonists can reduce acid secretion in the stomach, an effect that makes these drugs useful in managing various conditions, such as peptic ulcer disease.

Other conditions in which H-2 antagonists are used to lower gastric acidity include reflux esophagitis, stress ulcers, and hypersecretory states such as the Zollinger-Ellison syndrome, in which tumor cells secrete large amounts of the hormone gastrin, which stimulates gastric acid secretion. In these conditions, administration of H-2 antagonists reduces symptoms and promotes healing.

The toxicity of H-2 antagonists is quite low, and adverse effects are reported by only 1-2% of patients. The most common side effects are gastrointestinal upsets, including nausea, vomiting, and diarrhea.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


A drug that prevents or diminishes the effect of histamine; used in treating allergic reactions and common-cold symptoms.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


any drug that neutralizes the effects of histamine, used esp in the treatment of allergies
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
These studies showed that although montelukast was better than placebo in controlling the symptoms of allergic rhinitis, antihistamines were significantly more effective than montelukast.
They can also access the dual acting drugs, ie, ketotifen and olopatadine, as well as selective antihistamines (eg, azelastine and emedastine), all of which can be used in a similar manner to a mast cell stabilizer with certain caveats.
Meta-analyses show that oral antihistamines are not more effective than intranasal corticosteroids for eye symptoms.
And while such impairment may be less evident with many of the second-generation antihistamines when prescribed at the doses approved for seasonal allergic rhinitis, these agents are often used at far higher, even heroic doses in treating a variety of pruritic dermatologic diseases, Dr.
* Most antihistamines come in two strengths--a regular and an extra strength.
For that reason, UTMB researchers tested their theory that the use of an antihistamine or a corticosteroid--which is given to tamp down the immune system's response and thus reduce inflammation--would fight off the infections better.
Use of nonsedating antihistamines (including loratadine) and sedating antihistamines also were not associated with hypospadias.
Many health plans have put other antihistamine prescription allergy drugs on higher, more expensive pharmacy tiers to discourage members from choosing the prescription medications over Claritin.
* Antihistamines. Oral antihistamines counter the effects of histamines, chemicals released from mast cells and basophils, thus preventing allergic symptoms before they start.
Familiar over-the-counter antihistamines like Benadryl and newer, prescription ones like Claritin and Allegra address allergy problems at the far end of the process.
He just needed an antihistamine, but his luggage, with the antihistamines, had been lost.
Caffeine keeps flies awake and antihistamines, sleep-causing cold medicines, zonk them out.