[an-ti-his-tuh-meen, -min]
antihistamine, 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.

Synthetic drug that counteracts the effects of released histamine in the body. Antihistamines compete with histamine at one of the three types of histamine receptors, preventing allergic attacks (see allergy) or inflammation. Some antihistamines also prevent motion sickness and vertigo. Drowsiness is a frequent side effect. H2 antihistamines, which bind to the second receptor type, are used to control gastric-acid secretion (see stomach) and treat peptic ulcers.

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