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(ăn'tĭhĭs`təmēn), any one of a group of compounds having various chemical structures and characterized by the ability to antagonize the effects of histaminehistamine
, organic compound derived in the body from the amino acid histidine by the removal of a carboxyl group (COOH). Although found in many plant and animal tissues, histamine is specifically important in human physiology because it is one of the chemicals released from
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. 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.


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.


A drug that prevents or diminishes the effect of histamine; used in treating allergic reactions and common-cold symptoms.


any drug that neutralizes the effects of histamine, used esp in the treatment of allergies
References in periodicals archive ?
For many people, antihistamines provide relief from mild to moderate allergy symptoms.
Table 1: Distribution of patients according to sex Number of Percentage Sex Patients (%) Male 60 60% Female 40 40% Total 100 100 Table 2: Distribution of patients according to age Age of the Number of Percentage Patient Patients (%) 10-20 15 15% 21-30 40 40% 31-40 15 15% 41-50 25 25% 51-60 5 5% Table 3: Results Number RESULTS (Percentage) Treatment of Given Patients Good Satisfactory Poor Antihistamines + 25 50% 25% 25% Nasal Decongestants Antihistamines + 25 50% 30% 20% Nasal Decongestants + Corticosteroid Sprays Antihistamines+ 25 50% 35% 15% Nasal Decongestants + Mast cell stabilizers Specific Immunotherapy+ 25 80% 15% 5% Avoidance
The American Academy of Ophthalmology's 2012 evidence-based guideline recommends treating allergic conjunctivitis with topical antihistamines (Level A-1 evidence, defined as important evidence supported by at least one RCT or a meta-analysis) and using topical mast cell stabilizers if the condition is recurrent.
Second-generation antihistamines are selective for peripheral inactive [H.
If hay fever mainly affects your eyes, antihistamine eye drops are available from a pharmacist.
She usually takes Zyrtec, another one of the second generation of antihistamines.
A topical mast cell stabiliser has been recently added to the GSL category (as well as being available as a P-medicine), and while P-medicine topical ocular antihistamines have been discontinued, oral antihistamines are available as GSL products and as P-medicines.
The omalizumab group also made significantly less use of rescue antihistamines.
Antihistamines help prevent that bonding between histamines and other cells, thus preventing those annoying symptoms.
There are now more data on the positioning of oral anti-leukotriene agents as well as the intranasal antihistamines," said Dr.