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How do antihistamine drugs reduce the effects of histamine in the body?
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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.
Antihistamine drugs function by selectively counteracting the effects of histamine within the body. Histamine typically exerts its biological effects by binding to specific receptors located on the surface of target cells. There are four known types of these receptors, designated as H1, H2, H3, and H4.12
Antihistamines work through a process of competition: they replace histamine at these receptor sites, effectively blocking the histamine from binding and thereby preventing the subsequent histamine-triggered reactions.2
The specific therapeutic effect depends on which receptor the drug targets:
- H1 receptor antagonists: These drugs are primarily used to alleviate symptoms of allergic conditions, such as hay fever, itching, and hives, by competing with histamine for occupation of H1 receptors.12
- H2 receptor antagonists: These drugs, such as cimetidine, specifically block histamine from binding to H2 receptors in the stomach. This action inhibits the secretion of gastric acid and is used in the treatment of conditions like peptic ulcers.12