H2 Antagonist (blocker) Medications Definition

H2 Antagonist (blocker) Medications – medications that block molecular structures called histamine 2 receptors in the lining of the stomach. Histamines are chemicals called mediators that stimulate specific cells. Cells within the stomach’s lining, called enterochromaffin-like (ECL) cells, release histamine in response to the digestive hormone gastrin.

The histamine binds with H2 receptors on the parietal cells in the stomach. This binding stimulates the parietal cells to release hydrochloric acid into the stomach. H2 antagonists, or blockers, bind with the H2 receptors as well, blocking them from binding with endogenous histamine 2. The result is a decrease in acid production.

Other histamines are involved in different body functions. Histamine 1 (H1) stimulates smooth muscle contraction and immune response. H1 is familiar for its role in allergic response. Histamine 3 (H3) has neurotransmitter activity. Histamine receptors are primarily unique; only H1 receptors accept histamine 1, only H2 receptors accept histamine 2, and only H3 receptors accept histamine 3. The antihistamine medications to relieve allergy symptoms have no effect on gastric acid production. Similarly, the H2 blockers have no effect on allergies.

Doctors may prescribe H2 blockers to treat gastroesophageal reflux disorder (gerd)peptic ulcer disease, chronic gastritis, Crohn’s disease that involves the stomach and esophagus, hiatal hernia, and other conditions in which excessive gastric acid causes symptoms or tissue damage. The four H2 blockers used in the United States are available in over-the-counter (OTC) and prescription strengths.

Common H2 blockers

H2 BlockerPrescription StrengthOTC Strength
cimetidine (Tagamet)200mg, 300mg, 400mg, 800mg100mg
famotidine (Pepcid)20mg, 40mg10mg
nizatidine (Axid)150mg, 300mg75mg
ranitidine (Zantac)150mg, 300mg75mg

H2 blockers, most notably cimetidine, interact with numerous other medications. Antacids prevent the stomach from absorbing H2 blockers, significantly reducing the effectiveness of the H2 blocker. Side effects of H2 blockers may include dizziness, headache, and diarrhea. Changing to a different H2 blocker medication often resolves any side effects.


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