Gastroenterology · USMLE Step 1 and Step 2

Laxatives song

Laxatives - Types & Mechanisms, as a song. Hear the short teaser here. The full song is on YouTube and Spotify.

Laxatives

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Key points in this song10

Bulk-forming laxatives (e.g., psyllium) absorb water to increase stool volume and stimulate peristalsis; safest for long-term use and pregnancy.
Osmotic laxatives (e.g., lactulose, polyethylene glycol, magnesium hydroxide) draw water into the intestinal lumen by osmotic gradient, softening stool and promoting evacuation.
Lactulose is specifically used in hepatic encephalopathy because colonic bacteria convert it to organic acids, acidifying the colon and trapping NH₃ as NH₄⁺, reducing systemic ammonia.
Stimulant laxatives (e.g., bisacodyl, senna) directly stimulate the myenteric plexus to increase intestinal motility; long-term use can cause cathartic colon (loss of haustral folds).
Docusate is a stool softener (surfactant) that lowers surface tension, allowing water and fat to penetrate the stool; first-line for patients who must avoid straining (post-MI, post-hemorrhoidectomy).
Mineral oil acts as a lubricant laxative; chronic use impairs absorption of fat-soluble vitamins (A, D, E, K) and is contraindicated in aspiration-prone patients due to lipoid pneumonia risk.
Polyethylene glycol (PEG) solution is the standard bowel prep agent before colonoscopy; isotonic formulation minimizes electrolyte shifts.
Magnesium-containing laxatives are contraindicated in renal failure due to risk of hypermagnesemia causing hyporeflexia, bradycardia, and respiratory depression.
Methylnaltrexone and naloxegol are peripherally acting mu-opioid receptor antagonists (PAMORAs) used specifically for opioid-induced constipation without reversing central analgesia.
Chronic stimulant laxative abuse can cause melanosis coli (brown-black discoloration of colonic mucosa from lipofuscin deposits), a benign but diagnostically recognizable finding on colonoscopy.