How do H2 receptor antagonists reduce the secretion of stomach acid?

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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.

H2 receptor antagonists reduce the secretion of stomach acid by acting as competitive inhibitors at the histamine H2 receptors located on the parietal cells of the stomach.123

Under normal physiological conditions, histamine binds to these H2 receptors to stimulate the parietal cells to produce hydrochloric acid. By occupying these receptor sites, H2 receptor antagonists prevent histamine from binding, thereby blocking the signal that initiates acid production.1423

These drugs are also effective at blocking other stimulants of acid secretion, such as the hormone gastrin and food intake. By inhibiting this process, agents such as cimetidine and ranitidine help manage conditions characterized by the hypersecretion of gastric acid, including peptic ulcers and gastroesophageal reflux.15