Laxatives and Purging Agents

Laxatives Overview - The Gut Go-Getters

• Laxatives: Promote defecation; ease stool passage. Purgatives: Stronger, cause fluid evacuation.
• Constipation: Infrequent (<3/week), hard, or difficult-to-pass stools.
• Key Indications:
  • Functional constipation; Opioid-induced constipation (OIC)
  • Bowel prep (surgery/colonoscopy)
  • Hepatic encephalopathy (lactulose: ↓NH₃)
  • Avoid straining (post-MI, anorectal conditions)
• General Contraindications:
  • Undiagnosed abdominal pain, nausea, vomiting
  • Bowel obstruction, perforation, fecal impaction
• Classification:
  • Bulk-forming: Psyllium, Ispaghula (absorb water, ↑mass)
  • Osmotic: Lactulose, PEG, Mg salts (osmotic water retention)
  • Stimulant: Bisacodyl, Senna (↑motility, alter transport)
  • Stool Softeners/Lubricants: Docusates, Liquid paraffin (soften/lubricate)

⭐ Lactulose, a non-absorbable disaccharide, is key in hepatic encephalopathy by reducing ammonia production and absorption in the colon.

Bulk-Forming Agents - Gentle Giants

• Mimic dietary fiber; non-digestible hydrophilic colloids.
• Examples:
  • Psyllium (Ispaghula)
  • Methylcellulose
  • Calcium polycarbophil
  • Wheat dextrin
• Mechanism of Action (MOA):
  • Absorb water in gut → swell → form bulky, emollient gel.
  • ↑ Stool volume → distends colon → stimulates peristalsis.
  • Onset: 12-72 hours.
• Adverse Drug Reactions (ADRs):
  • Bloating, flatus.
  • ⚠️ Esophageal/intestinal obstruction or impaction if taken with insufficient fluid.
• Clinical Uses:
  • First-line for chronic constipation.
  • Irritable Bowel Syndrome with Constipation (IBS-C).
  • Diverticulosis.
  • Safe in pregnancy & for long-term use.

⭐ Psyllium can also bind bile acids, leading to a modest reduction in LDL cholesterol. 📌 "Bulk up with water, or you'll be stuck!"

Osmotic Laxatives - Water Wonders

These agents draw water into the bowel, softening stool and promoting motility.

• MOA: Create osmotic gradient → ↑ intraluminal water → stimulate peristalsis.
• Types & Examples:
  • Saline:
    • Magnesium Salts: Sulfate (Epsom), Hydroxide (Milk of Magnesia), Citrate.
    • Sodium Phosphate (⚠️ Nephropathy risk).
  • Non-absorbable Sugars/Alcohols:
    • Lactulose: Disaccharide.
    • Sorbitol, Mannitol.
    • Polyethylene Glycol (PEG): Large polymers. 📌 "PEG pulls H₂O!"
• Key Uses:
  • Lactulose: Hepatic encephalopathy (↓ $NH_3$).
  • PEG: Bowel prep (colonoscopy), chronic constipation.
  • Saline laxatives: Rapid evacuation.
• ADRs:
  • Electrolyte imbalance (e.g., ↑$Mg^{2+}$, ↑$PO_4^{3-}$, ↓$K^+$).
  • Dehydration, cramping, flatulence.

⭐ Lactulose, metabolized by gut flora to acidic byproducts, lowers colonic pH, trapping ammonia as NH3 (converting to NH4+), thus reducing systemic ammonia levels in hepatic encephalopathy.

Stimulants & Softeners - Active Accelerators

• Stimulants (Irritants): ↑ intestinal motility via nerve stimulation.
  • Anthraquinones (Senna, Cascara):
    • MOA: Hydrolysed to emodin; stimulates colon.
    • ADRs: Melanosis coli (reversible), cathartic colon (prolonged use).
  • Diphenylmethanes (Bisacodyl, Sod. Picosulfate):
    • Bisacodyl: Enteric-coated (avoid antacids/milk).
    • Sod. Picosulfate: Prodrug, colonic bacteria activate.
    • ADRs: Cramps, electrolyte imbalance.
  • Castor Oil:
    • MOA: Hydrolysed to ricinoleic acid (irritant). Acts on small intestine.
    • ADRs: Uterine contraction (⚠️ CI: Pregnancy).
• Stool Softeners & Lubricants:
  • Docusates (DOSS):
    • MOA: Anionic surfactant; softens stool.
    • Use: Prevent straining.
  • Liquid Paraffin (Mineral Oil):
    • MOA: Lubricates feces.
    • ADRs: Lipid pneumonitis (aspiration), ↓ fat-soluble vitamin absorption.
• Newer Agents: Lubiprostone (ClC-2), Linaclotide (GC-C), Prucalopride (5-HT₄).

⭐ Bisacodyl should not be taken within 1 hour of antacids or milk as these can disrupt its enteric coating, leading to gastric irritation.

High‑Yield Points - ⚡ Biggest Takeaways

• Bulk-forming laxatives (e.g., psyllium) mimic fiber; require adequate fluids.
• Osmotic laxatives (e.g., lactulose, PEG) draw water into colon; lactulose reduces ammonia in hepatic encephalopathy.
• Stimulant laxatives (e.g., senna, bisacodyl) increase motility; chronic use risks cathartic colon.
• Stool softeners (e.g., docusate) facilitate water mixing, preventing straining.
• Lubiprostone & Linaclotide are for chronic constipation and IBS-C.
• Opioid-induced constipation treated with PAMORAs like methylnaltrexone.
• Avoid castor oil in pregnancy due to uterine stimulation.