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Why were early aniline-type antihistamine compounds not used clinically?
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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.
Early aniline-type antihistamine compounds were not used clinically because they were found to be too toxic for human use, despite being more potent than the initial ethylamine derivatives.1