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Antihistamines are inhibitors of histamine receptors. H1-antihistamines, inhibit competitively H1 receptors and the corresponding effects i. They do not inhibit antigen/antibodies reactions, nor histamine release, they inhibit H1 effects. H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit, by inhibiting stimulant effect of histamine, drowsiness.Azelastine is a H1-antihistamine usable by local route, in nasal pulverization, in the treatment of allergic rhinitis.H1-antihistamines are used for supportive care of allergic manifestations, cutaneous (urticaria) or mucous membranes (rhinitis, hayfever, conjunctivitis). Insufficient alone to treat anaphylactic shock or edema of the larynx, they could prevent them.
They were withdrawn from the market because they could induce ventricular arrhrythmias, prolongation of QT interval which can lead to torsades de pointes.
Certain H1 antagonists, such as promethazine, have a local anesthetic effect.
Other H1-antihistamines having an important antimuscarinic activity like diphenhydramine and dimenhydrinate, are used in preventive and curative treatment of motion sickness, but scopolamine which has no antihistamine effect seems more effective than them in preventive treatment.
New H1-antihistamines, terfenadine and astemizole, which were associated with cardiac adverse effects, prolongation of QT interval, torsades de pointes by inhibition of potassium channels, are no longer approved for use. Other adverse effects of H1-antihistamines have been reported, in particular allergic reactions.
All H1-antihistamines, including those which are intended for treatment of motion sickness, are disadvised during the first three months of pregnancy, more for reasons of principle than for observations of malformations.
New H1-antihistamines induce only exceptionally drowsiness. This possibility, even rare, must however be taken into account, particularly at time of a first prescription.