Osmotic Laxatives: Lactulose and Magnesium Hydroxide
Both lactulose and magnesium hydroxide are osmotic laxatives, while bisacodyl and senna are stimulant laxatives. 1
Mechanism-Based Classification
The American Gastroenterological Association clearly categorizes laxatives by their mechanism of action 1:
Osmotic Laxatives
- Polyethylene glycol (PEG) - draws water into intestine to hydrate and soften stool 1
- Lactulose - semi-synthetic disaccharide that produces osmotic diarrhea 1, 2
- Magnesium citrate - osmotic agent 1
- Magnesium hydroxide - osmotic agent 1
Stimulant Laxatives
- Bisacodyl - irritates sensory nerve endings to stimulate colonic motility and reduce colonic water absorption 1
- Senna - stimulates myenteric plexus and inhibits colonic water absorption 1, 2
- Sodium picosulfate - prodrug metabolized to stimulant 1
Clinical Context
For chronic constipation, osmotic laxatives (PEG, lactulose, magnesium hydroxide) are recommended as first-line therapy over stimulant laxatives. 1, 2
- PEG is the preferred osmotic agent due to superior tolerability compared to lactulose, which commonly causes flatulence, bloating, and abdominal discomfort 1, 2, 3
- Magnesium-containing osmotic laxatives should be used cautiously in renal impairment due to risk of hypermagnesemia 1
- Stimulant laxatives like bisacodyl and senna are typically reserved for short-term use, rescue therapy, or when osmotic agents provide inadequate response 1, 2
Common pitfall: Confusing mechanism of action with clinical indication—while both osmotic and stimulant laxatives treat constipation effectively, their mechanisms differ fundamentally (water retention versus motility stimulation). 1