Medicine & USMLE

H2 Blockers (Cimetidine, Ranitidine, Famotidine)

Gastrointestinal Pharm
  1. H2 Blockers (Cimetidine, Ranitidine, Famotidine)
  2. Proton Pump Inhibitors (PPIs)
  3. Misoprostol
  4. Orlistat
  5. Antacids
  6. Bismuth & Sucralfate
  7. Octreotide
  8. Diphenoxylate vs. Loperamide
  9. Ondansetron
  10. Metoclopramide
  11. Bulk-forming Laxatives
  12. Senna
  13. Docusate
  14. Aprepitant


H2 receptor blockers or H2 antagonists are drugs that block the action of histamine at H2 receptors in the stomach. This drug class includes cimetidine, ranitidine, and famotidine. Blocking histamine at H2 receptors notably decreases stomach acid production, which can be used to treat acid-mediated disorders like peptic ulcer disease and gastroesophageal reflux disease. Importantly, the drug cimetidine has a few notable side effects. Cimetidine is a powerful CYP enzyme inhibitor that can lead to various drug interactions. Cimetidine also has anti-androgenic effects, which can lead to gynecomastia.

Key Points

  • H2 Blockers
    • Drug Names
      • Cimetidine
      • Ranitidine
      • Famotidine
      • Nizatidine
    • Mechanism of Action
      • Causes a reversible block of H2 Receptors → Decreased H+ Secretion from Parietal Cells
    • Clinical Use
      • Treats Peptic Ulcers/Gastritis
        • Treats gastroesophageal reflux and Peptic Ulcer Disease (PUD) by blocking parietal cell H2 receptors.
        • Treats Zollinger-Ellison Syndrome (severe PUD)
      • Treats GERD
        • H2 blockers (eg, ranitidine) reduce gastric acid secretion and can alleviate symptoms of gastroesophageal reflux
    • Adverse Effects:
      • Cimetidine-specific side effects:
        • Acts as a Cytochrome P450 inhibitor (multiple drug interactions)
          • From nathan: This is SUPER high yield. They will get you via warfarin or theophylline interaction
        • Causes Antiandrogenic effects (Prolactin Release, Gynecomastia, Impotence, Decreased Libido)
        • Can Cross BBB → Confusion, Dizziness, Headaches
        • Can Cross Placenta
      • Cimetidine and Ranitidine decrease renal excretion of creatinine
      • Other H2 blockers are relatively free of these effects