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How do newer H1 antihistamines cause less drowsiness than older ones?
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antihistamine
Antihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H1 receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.
Newer H1 antihistamines cause less drowsiness than older versions primarily because they do not cross the blood-brain barrier effectively.1
Older H1 antihistamines are known to cause drowsiness because they act on the central nervous system, where they bind to H1 receptors and often muscarinic receptors in the brain. In contrast, the chemical structure of newer antihistamines prevents them from entering the brain in significant amounts, thereby avoiding the sedative effects associated with central nervous system depression.1