H2-receptor antagonists are drugs that block the histamine receptor on parietal cells. Recall that histamine, gastrin, and acetylcholine stimulate parietal cells to secrete acid and that histamine has the greatest stimulatory effect. H2 receptors are the type of histamine receptor on parietal cells. Cimetidine (Tagamet), ranitidine (Zantac), and famotidine (Pepcid) are H2-receptor antagonists than bind reversibly to H2 receptor. They suppress 24 hour gastric acid secretion approximately 70%. The H2 receptors are different from the H1 receptors involved in allergic receptors. Antihistamines that block allergic reactions do not affect histamine-mediated gastric acid secretion, and vice versa.