Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells, and neurons.
Its various actions are mediated by histamine receptors H1, H2, H3, and H4. The histamine receptor H2 stimulates gastric acid secretion, regulates gastrointestinal motility and intestinal secretion, and is involved in regulating cell growth and differentiation.

H2 antagonists, also called H2 blockers, are a class of medications that block the action of histamine at the histamine H2 receptors of the parietal cells in the stomach; this decreases the production of stomach acid. They can be used in the treatment of dyspepsia, but have been surpassed by the more effective proton pump inhibitors.

The H2 antagonists, cimetidine, ranitidine, famotidine, and nizatidine are gastroprotective drugs that reduce both gastric acid output and the volume of gastric secretions.

Notes:
- Cimetidine is associated with potentially serious drug interactions. It reduces the absorption of antifungal drugs (e.g., ketoconazole, itraconazole) and, via cytochrome P450, may increase the plasma levels of anti-epileptics, benzodiazepines, warfarin, antidepressants, and other drugs.
- H2 antagonists may improve symptoms and prevent duodenal ulceration in patients taking NSAIDs, but do not prevent gastric ulceration.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on October 25, 2016.