Medicine & USMLE

Osmotic Laxatives (Lactulose, PEG)

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


Osmotic laxatives include the drugs lactulose, polyethylene glycol or PEG, magnesium hydroxide, and magnesium citrate. As laxatives, these drugs are used to increase bowel movements to help treat constipation, as well as to prepare the bowel for surgery or diagnostic procedures. In addition, lactulose can be used to treat hepatic encephalopathy by lowering ammonia levels.

Key Points

  • Osmotic Laxatives
    • Key Drugs
      • Lactulose
      • Polyethylene glycol / PEG (Miralax)
        • A large volume of solution (approx. 4L) to be consumed over 3 hours
      • Saline laxatives
        • Magnesium hydroxide (milk of magnesia)
        • Magnesium citrate
    • Mechanism
      • Pull water into the stool through osmosis. The increased water stimulates the stretch receptors in the intestine, causing peristalsis and bowel evacuation
    • Clinical Uses
      • Constipation
        • Given in low doses
        • Prevents pain and straining in patients with episiotomy or hemorrhoids
      • Hepatic encephalopathy (lactulose)
        • Lowers ammonia levels by reducing intestinal ammonia absorption
      • Pre-surgery / diagnostic imaging bowel preparation
        • Barium enema, colonoscopy
        • Given in large doses
    • Side Effects and Adverse Reactions
      • Electrolyte imbalances
        • Hypermagnesemia may occur in patients with renal disease taking magnesium laxatives
      • Dehydration
        • Encourage patient to increase fluid intake