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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 ?
39) In another study, van Cauwenberge et al (30) assessed the impact of once-daily fexofenadine HCl 120 mg, loratadine 10 mg and placebo (n = 509) on the QoL of patients with SAR.
A randomized, double-blind, parallel-group study of 96 children aged 3 to 6 years with seasonal allergic rhinitis showed that the therapeutic response at the study's endpoint was rated by physicians as good or excellent in 82% of children who took loratadine syrup at 5 or 10 mg once daily for 14 days, compared with only 60% of those who took terfenadine suspension at 15 mg twice daily for 14 days.
While the adverse effects of loratadine were being read by the announcer, there was a competing message on the screen and the description of the drug's adverse reactions were read so quickly that it was difficult for the typical consumer to understand.
which markets loratadine, says the company's own research shows no evidence that the drug promotes malignant tumor growth.
The uncertainty regarding the reproductive safety of loratadine was complicated by the publication of a retrospective analysis from Sweden in 2002 suggesting that the risk of hypospadias in babies exposed in utero to loratadine was twice that of the general population.
A comparison of the efficacy of fluticasone propionate aqueous nasal spray and loratadine, alone and in combination, for the treatment of seasonal allergic rhinitis.
Certain statements in this release are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, including statements regarding Taro's ciclopirox nail lacquer and loratadine syrup products.
Over-the-counter loratadine is very costly for patients who formerly had it covered by insurance, said Dr.
Safety and efficacy of loratadine (Sch-29851): A new non-sedating antihistamine in seasonal allergic rhinitis.
Each drug is Consumer Reports Health's recommended choice within its category: acetaminophen for fever reduction and pain relief; loratadine for seasonal allergies; and omeprazole for heartburn.
The companies also terminated the Schering-Plough/Merck Pharmaceuticals respiratory joint venture, which was formed in May 2000 to develop and market a fixed-combination product that would combine loratadine and montelukast.
Benadryl[R]), and the newer antihistamines which are considered less sedating, such as loratadine (e.