SUPERStudy: H2 Receptor Blockers

By usmlevideoss

Introduction: H2 receptor blockers are a class of medications that reduce stomach acid production by blocking histamine type 2 (H2) receptors on the gastric parietal cells. They are commonly used to treat acid-related gastrointestinal conditions such as ulcers, gastroesophageal reflux disease (GERD), and dyspepsia.

Name of the Drug Class: H2 Receptor Antagonists (H2RAs)

Drugs in the Class

  1. Cimetidine
  2. Ranitidine (withdrawn in some countries due to safety concerns related to NDMA impurities)
  3. Famotidine
  4. Nizatidine

Mechanism of Action

  • H2 receptor blockers, also known as H2 antagonists, inhibit the action of histamine at H2 receptors on the parietal cells in the stomach lining. Normally, histamine binds to these receptors, stimulating the production of gastric acid via activation of adenylate cyclase and increasing cyclic AMP, which drives the proton pump (H+/K+-ATPase). By competitively blocking histamine from binding to H2 receptors, these drugs decrease cyclic AMP levels, thereby reducing the activity of the proton pump and subsequent secretion of gastric acid. 
  • Unlike proton pump inhibitors (PPIs), H2 blockers do not directly inhibit active proton pumps.

Clinical Uses

  1. Gastroesophageal Reflux Disease (GERD): Relief of mild-to-moderate symptoms.
  2. Peptic Ulcer Disease (PUD): Promotes healing of gastric and duodenal ulcers.
  3. Zollinger-Ellison Syndrome: Reduces acid secretion in patients with gastrin-secreting tumors.
  4. Dyspepsia: Alleviates indigestion and heartburn.
  5. Prophylaxis: Prevents stress ulcers in critically ill patients.

Administration

  • Route: Oral or intravenous.
  • Dosing: Typically taken at bedtime or before meals for maximum effect, as nocturnal acid secretion is most dependent on histamine stimulation.

Side Effects

  1. Common Side Effects:
    • Headache
    • Diarrhea or constipation
    • Dizziness
    • Fatigue
  2. Specific Side Effects:
    • Cimetidine: Gynecomastia, impotence, galactorrhea (due to anti-androgenic effects).
    • Rare but serious: Confusion, especially in elderly or renal-impaired patients.
  3. Drug Interactions:
    • Cimetidine inhibits cytochrome P450 enzymes, affecting metabolism of drugs like warfarin, theophylline, and phenytoin.

Superpoint: H2 receptor blockers reduce gastric acid secretion by blocking histamine type 2 receptors on parietal cells and are used for conditions like GERD, peptic ulcers, and stress ulcer prophylaxis, with famotidine being the preferred agent due to fewer side effects and drug interactions.

SUPERFormula: 

it blocks H2 receptors on parietal cells + Decreases gastric acid secretion + Used for GERD, peptic ulcers, and Zollinger-Ellison syndrome + Side effects include gynecomastia, galactorrhea, and confusion in elderly + Inhibits cytochrome P450 enzymes, causing drug interactions = Cimetidine