Medicine & USMLE

H2 Blockers

GI Drugs
  1. Ondansetron
  2. Sucralfate
  3. Docusate (Stool Softener)
  4. Bulk Forming Laxatives (Psyllium, Methylcellulose)
  5. Osmotic Laxatives (Lactulose, PEG)
  6. Stimulant Laxatives (Senna, Bisacodyl)
  7. Antidiarrheals (Loperamide, Diphenoxylate-Atropine)
  8. Bismuth Subsalicylate
  9. Antacids
  10. Proton Pump Inhibitors (PPIs)
  11. H2 Blockers
  12. Metoclopramide
  13. Misoprostol
  14. Orlistat
  15. Octreotide


Histamine receptor type 2 antagonists, also called H2 blockers, are a class of drugs easily recognizable by their “-tidine” ending, such as cimetidine, ranitidine, and famotidine. These drugs reduce the production of gastric acid, making them useful in treating GI ulcers and gastroesophageal reflux disease or GERD. H2 blockers can cause hepatotoxicity, so keep a close eye on the patient’s liver enzymes. Cimetidine can also cause several drug interactions.

Key Points

  • Histamine Receptor 2 (H2) Blockers
    • Also called H2 Receptor Antagonists
    • Key Drugs
      • “-tidine” Ending
        • Cimetidine (Tagamet)
        • Ranitidine (Zantac)
        • Nizatidine (Axid)
        • Famotidine (Pepcid)
    • Mechanism
      • Block H2 receptors in the parietal cells, reducing gastric acid secretion
        • Different than antihistamines which block H1 receptors to treat allergic conditions
    • Clinical Use
      • Ulcers
        • Gastric and duodenal
        • Treatment and prophylaxis
      • Gastroesophageal Reflux Disease (GERD) / Heartburn
        • To be given at bedtime
    • Side Effects and Adverse Reactions
      • Hepatotoxicity
        • Watch for signs of jaundice and monitor liver enzymes
      • Drug interactions (cimetidine)
        • Cimetidine can inhibit the CYP450 liver enzymes used to metabolize many other drugs
        • Can increase serum theophylline levels
      • Impotence, gynecomastia
        • When given over a long period of time
      • Headache
      • Confusion, Agitation
      • Dizziness
      • Nausea