Medicine & USMLE


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


Antacids are drugs used to neutralize acid in the stomach and GI tract. This neutralizing action helps prevent acid-mediated damage in conditions like peptic ulcer disease and acid reflux. Common antacids to know include magnesium hydroxide, aluminum hydroxide, and calcium hydroxide. While magnesium hydroxide can lead to wet feces and diarrhea, aluminum hydroxide can lead to constipation. Calcium carbonate can cause hypercalcemia in high doses. All antacids should not be taken with fluoroquinolones, as they can bind to and prevent the GI absorption of fluoroquinolones taken orally.

Key Points

  • Antacids
    • Drug Names:
      • Aluminum hydroxide (Al(OH)3)
      • Calcium carbonate (CaCO3)
      • Magnesium hydroxide (Mg(OH)2)
    • Mechanism:
      • Neutralizes acid (antacid)
        • Increases gastric pH
      • Delays gastric emptying
    • Clinical Use:
      • Reduce acidic damage on stomach and esophageal epithelium
        • Acid reflux
        • Peptic Ulcer Disease
    • Adverse effects:
      • Causes Metabolic Alkalosis
        • Can result in hypokalemia, due to K+/H+ exchange across cell membranes to lower pH
      • Reduces Oral Absorption of Fluoroquinolones
        • Can affect absorption, bioavailability, or urinary excretion of other drugs by altering gastric and urinary pH or by delaying gastric emptying
          • ↑ absorption of weak bases
          • ↓absorption of weak acids
      • AlOH
        • Constipation 
        • Hypophosphatemia
        • Osteodystrophy
        • Proximal muscle weakness
        • Seizures
      • CaCO3
        • Hypercalcemia (milk-alkali syndrome)
          • Calcium carbonate is used as a supplement for insoluble calcium
        • Rebound acid increase
      • MgOH
        • Diarrhea
          • Magnesium hydroxide (milk of magnesia) is also used as a osmotic laxative
        • Hyporeflexia
        • Hypotension
        • Cardiac arrest