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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 ?
Participants started taking cetirizine (10mg) orally daily and continued for one year, in addition to their established NMOSD disease-modifying therapy.
Waitrose sells a 30 pack of Clarityn containing 10mg of cetirizine dihydrochloride for PS9.
It was also observed that cetirizine was the most prescribed antihistamine for all the disorders except herpes zoster where levocetirizine was prescribed more than cetirizine (Table 2).
AC-170 is a formulation of cetirizine (an oral antihistamine, available under brand names including Zyrtec) being assessed for the treatment of allergic conjunctivitis.
Just recently, Food and Drug Administration (FDA) associated cetirizine with potential signs of severe risk for oculogyric crisis.
Keeping in view of the above ideas, the present study has been undertaken to evaluate the anti-inflammatory activity of newer non-sedative H1 receptor blocker cetirizine on the acute and chronic inflammatory models in albino rats.
In this clinical trial, the oral Ht antihistamine cetirizine hydrochloride, available only by prescription when this study was conducted, was tested to determine whether this agent could reduce or prevent the symptoms of LISRs.
There is one OTC 1st generation antihistamine available for children, diphenhydramine (sold as Benadryl and other names), and two 2nd generation medications, loratadine (Claritin) and cetirizine (Zyrtec).
FDA has given final approval to Perrigo Company to manufacture and market over-the-counter (OTC) Children's Cetirizine Hydrochloride Oral Solution (Allergy) and Children's Cetirizine Hydrochloride Oral Solution (Hives-Relief), 1mg/mL.