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Laxative overview

Bulk-forming laxatives

  • Examples: Psyllium husk powder, methylcellulose, ispaghula husk, bran
  • Mechanism:
    • Contain indigestible hydrophilic polysaccharides
    • Absorb water in the colon → swell to form a bulky, gel-like stool
    • Increased bulk stretches the colonic wall → stimulates mechanoreceptors → triggers peristalsis
  • Time to effect: 1–3 days
  • Notes:
    • First-line for chronic constipation
    • Require adequate hydration to avoid impaction

Osmotic laxatives

  • Examples:
  • Mechanism:
    • Osmotically active substances remain in the bowel lumen → retain water by osmosis → increase stool volume and softness
    • Large stool volume → triggers peristalsis
  • Time to effect: 0.5–6 hours (saline), 1–3 days (lactulose, PEG)
  • Notes:
    • Can cause bloating and electrolyte imbalance (esp. Mg, Na, phosphate) in renal impairment
    • Used for bowel prep at higher doses

Stimulant laxatives

  • Examples: Senna, Bisacodyl, Sodium picosulfate, castor oil
  • Mechanism:
    • Directly stimulate enteric nerve plexus (Auerbach’s and Meissner’s plexuses) → increases colonic motility
    • May also increase secretion of water and electrolytes into the lumen
  • Time to effect: 6–12 hours (oral), 15–60 min (rectal)
  • Notes:
    • Risk of cramping, diarrhea
    • Chronic use → melanosis coli, dependence

Stool softeners (emollients)

  • Examples: Docusate sodium, docusate calcium
  • Mechanism:
    • Anionic surfactants reduce surface tension → allow water and lipids to mix into stool → softens stool
  • Time to effect: 1–3 days
  • Notes:
    • Mild effect; best for prevention (e.g., post-hemorrhoidectomy)
    • Less effective in established constipation

Lubricant laxatives

  • Examples: Liquid paraffin (mineral oil), Glycerol suppository
  • Mechanism:
    • Coats fecal material → reduces water absorption from stool → eases passage
    • Lubricates mucosa → reduces friction
  • Time to effect: 6–8 hours (oral), 15–60 min (rectal)
  • Notes:
    • Risk of lipid pneumonia (aspiration of oil) → avoid in elderly or dysphagia
    • Can interfere with absorption of fat-soluble vitamins

Enemas

  • Phosphate enema (Fleet®):
    • Rapid action (5–20 minutes), effective for acute constipation; caution in renal dysfunction and elderly (risk of phosphate retention, electrolyte imbalance)
    • Hyperosmotic - Draw water into the colon → soften stool and increase motility
  • Micro-enema (Microlax®)
    • Active ingredients: sodium citrate (osmotic), sodium lauryl sulfoacetate (wetting agent), sorbitol
    • Mechanism: draws water into the bowel lumen → softens stool + mild stimulation of peristalsis
    • Onset: usually effective within 5–15 minutes

Mixed

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