Why are H1 antihistamines not usually effective for asthma?

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antihistamines, including Benadryl — 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.

H1 antihistamines are generally ineffective for treating asthma because histamine is not the primary agent responsible for producing the symptoms associated with the condition.1

While histamine is a chemical messenger capable of stimulating the contraction of smooth muscle in the bronchi, the limited efficacy of H1-blocking agents in asthma indicates that other factors play a more significant role in the underlying pathology of the disease.1