Histamine is a body chemical that is released in response to digestion in the stomach or as an immune response, such as an allergic reaction. Usually, these types of body reactions are self-limiting and in response to a need. However, blockade of histamine activity can be useful for disorders such as continual seasonal allergies or stomach ulcers. Most types of histamine blockers work on histamine receptors by joining to the receptor so that histamine cannot. There are two common types of histamine receptors and two corresponding classes of histamine-blocking drugs.
First-Generation H1 Histamine Receptor Blockers
H1 histamine receptors are found in smooth muscle cells throughout the body, and they lead to a traditional allergic reaction when histamine is bound to them. The first generation of histamine blockers, antihistamines, was developed to counteract the allergic symptoms. The most common example of a first-generation histamine blocker is diphenhydramine (Benadryl). It and other first-generation antihistamines have a documented side effect of extreme sedation.
Second-Generation H1 Histamine Receptor Blockers
Because of the sedative effect of first-generation histamine blockers, second-generation versions of H1 histamine receptor blockers were developed and marketed in the 1990s. As of 2010, these second-generation histamine receptor blockers are still commonly used in over-the-counter and prescription products and include loratadine (Claritin) and fexofenadine (Allegra). Because of the changed chemical structure of second-generation histamine blockers, less sedation is typically present.
H2 Histamine Receptor Blockers
H2 histamine receptors bind with histamine just as H1 receptors do, but H2 receptors are found in the stomach lining. Stimulating these receptors causes increased digestion and stomach acid secretion. H2 receptor blockers, such as famotidine (Pepcid) and ranitidine (Zantac), are also available over the counter and as prescriptions. These types of histamine blockers are used to counteract excess stomach acid in peptic ulcer disease or gastroesophageal reflux disease (GERD).