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Why do some antihistamines cause drowsiness?
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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.
Some antihistamines cause drowsiness because they act on the central nervous system. Specifically, older H1 receptor antagonists are capable of crossing the blood-brain barrier, where they bind to H1 receptors and muscarinic receptors in the brain. This interaction produces a sedative effect, which is why some of these drugs are utilized as sleep aids or to treat motion sickness.12
In contrast, newer H1 antihistamines are designed not to cross the blood-brain barrier, making them relatively free of this sedative side effect. Because older antihistamines can impair performance—such as when driving—and may enhance the effects of alcohol or other central nervous system depressants, it is important to be aware of these risks when using them.1