Medicine & USMLE


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


Antacids are a class of drugs that include calcium carbonate, magnesium hydroxide, aluminum hydroxide, and sodium bicarbonate. These drugs work by neutralizing stomach acid, which is useful for treating GI ulcers, as well as gastroesophageal reflux disease or GERD. The side effects of antacids depend on the specific drug taken: Aluminum hydroxide and calcium carbonate cause constipation, while magnesium hydroxide causes diarrhea. Sodium bicarbonate can increase sodium levels to cause fluid retention.

Key Points

  • Antacids
    • Key Drugs
      • Calcium carbonate (Tums)
      • Magnesium hydroxide (Milk of magnesia)
      • Aluminum hydroxide
        • Best choice of antacid for patients in renal failure. In renal failure, the kidneys can’t clear out calcium- or magnesium-based antacids very well and can lead to toxic levels.
      • Sodium bicarbonate
        • Now seldom used because of its systemic side effects
    • Mechanism
      • Neutralizes stomach acid and reduces pepsin activity
    • Clinical Uses
      • Ulcers
      • Gastroesophageal Reflux Disease (GERD)
      • Osteoporosis
        • Calcium carbonate may be used as a calcium supplement for patients with osteoporosis
      • Metabolic acidosis
        • The acid neutralizing features of sodium bicarbonate are effective in reversing metabolic acidosis.
    • Side Effects and Adverse Reactions
      • Constipation (aluminum hydroxide and calcium carbonate)
      • Diarrhea (magnesium hydroxide)
        • Also used as a laxative (also known as milk of magnesia)
      • Fluid/sodium retention (sodium bicarbonate)
        • Should be avoided in patients with hypertension or fluid overload
      • Metabolic alkalosis
        • Antacids are bases and when taken in high doses, may be absorbed in sufficient amounts to increase the pH of the blood
      • Hypophosphatemia (aluminum hydroxide)
        • Can lower phosphate levels