How do oral laxatives work?

Lawrence S. Friedman, MD
Depending on the type, oral laxatives work in a variety of ways to ease the passage of stool through the rectum.

Bulk-forming agents. These fiber-based products take a day or so to work but are very effective and safe to take indefinitely on a daily basis. Take with plenty of liquid.
  • bran (in food and supplements)
  • calcium polycarbophil (e.g., FiberCon)
  • methylcellulose (Citrucel and others)
  • psyllium (Metamucil and other brands)
Stool softeners merge with feces and soften consistency.
  • docusate (Colace, Surfak, others) -- generally safe for long-term use
  • mineral oil -- avoid daily use because it reduces absorption of fat-soluble vitamins and can cause lung damage if accidentally inhaled
Osmotic agents are salts or carbohydrates that promote secretion of water into the colon. They are reasonably safe, even with prolonged use.
  • polyethylene glycol (Miralax) -- shown to be helpful in children and adults with functional constipation
Saline laxatives attract and retain water in the intestines, increasing pressure and release of stool.
  • Magnesium (Milk of Magnesia, Epsom Salt, Fleet Phospho Soda, Phillips)
Stimulant laxatives act directly on the intestinal lining to alter water and electrolyte secretion. They're best used for occasional constipation.
  • bisacodyl (Dulcolax, Ex-Lax Ultra, Correctol, and others)
  • casanthranol (included in Dialose Plus, Peri-Colace)
  • cascara (included in Naturalax)
  • castor oil (Purge)
  • senna (Ex-Lax, Fletcher's Castoria, Senokot, others) A unique side effect of some stimulant laxatives, those in the class known as anthraquinones (casanthranol, cascara, senna), is pseudomelanosis coli -- a darkening of the lining of the colon seen on colonoscopy. However, pseudomelanosis coli is not associated with altered colon function and appears to be a harmless consequence of long-term stimulant laxative use.
Chloride-channel agonists cause additional fluid to be secreted into the intestine, making it easier to pass stool.
  • lubiprostone (Amitiza)

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.