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Which histamine receptors are blocked by antihistamines used for allergies?

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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.

Antihistamines used to treat allergies specifically block H $_{1}$ receptors.12

These drugs, known as H $_{1}$ receptor antagonists (or H $_{1}$ -blocking agents), function by competing with histamine for occupation of these receptor sites, thereby preventing the allergic symptoms triggered by histamine release. While there are four types of histamine receptors in the body (H $_{1}$ , H $_{2}$ , H $_{3}$ , and H $_{4}$ ), conventional allergy medications target the H $_{1}$ receptors.12

In contrast, H $_{2}$ receptor antagonists are a different class of drugs used to inhibit gastric acid secretion and treat conditions such as peptic ulcers.12