H1-antihistamines are inverse agonists that combine with and stabilize inactive conformation of H1-receptors.
Thus they interfere with actions of histamine at H1-receptors. They are widely used for treatment of allergic rhinitis, allergic
conjunctivitis, urticaria, coughs, colds and insomnia. H1-antihistamines are classified as older ‘first generation’ and
newer ‘second generation’. First generation H1-antihistamines have poor receptor H1-receptor selectivity, and cross
blood-brain-barrier. They have a lot of adverse events such as anti-muscarinic, anti-α-adrenergic, anti-serotonin, and sedative
effects. In contrast, second generation H1-antihistamines were highly selective for the histamine H1-receptor, do not
cross the blood brain barrier, and have minimal adverse events. The risks of first-generation H1-antihistamines have been
clearly underestimated, particularly when purchased as nonprescribed over the counter medications by public. This review
summarizes curent literature to evaluate antihistamines including their mechanism, indications and side-effects.
Keywords: Adverse events, antihistamine(s), allergic rhinitis, atopic dermatitis, cardiotoxicity, histamine, pharmacology,
receptor, sleep, urticaria
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