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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 ?
A potential drug interaction, resulting in stillbirth, occurred when a mother took 50 mg of the antihistamine for itching and then, 1.
Antihistamines are very effective treatments for allergic rhinitis and have very good effects on rhinorrhoea, sneezing and itching.
It may take a week or so before a reasonable tolerance to allergens is achieved, thus the potential need for eye washes, decongestants or topical ocular antihistamines to initially manage these conditions.
Intranasal antihistamines can be just as effective as oral second-generation antihistamines for seasonal allergic rhinitis, but they generally are less effective than intranasal corticosteroids for seasonal and perennial allergic rhinitis.
The often-cited distinction between first- and second-generation antihistamines is mostly marketing hype.
Continue taking the antihistamine and NSAID every 12 hours until the cold symptoms clear (3 to 7 days).
Many of these classic antihistamines are currently available over-the-counter (OTC), are relatively inexpensive, and, consequently, are widely used by patients.
Decongestants can cause dizziness, while antihistamines taken for hay fever can cause delayed reaction times.
I always go out with a bag full of tissues and If I'm going to a wedding at the weekend for example, I'll start taking my antihistamines early in the week.
There are some new antihistamines which are very effective, good ones if you are going away on holiday, some which you just need to take once a day.
First generation--with a long history in the market, first generation antihistamines are effective, but can cause drowsiness;
Although pyrilamine is an over-the-counter antihistamine, it's premature to say that people taking antihistamines for allergies may have trouble regulating their body temperature in heat stress, Wong cautions.