PharmacologyTier 2Medical Sciences concept

Laxative mechanisms of action (osmotic, stimulant, bulk-forming)

Core concept

Class	Example	Mechanism
Osmotic	Lactulose, macrogol/PEG, magnesium salts	Non-absorbed solute draws water into the lumen osmotically
Stimulant	Senna, bisacodyl	Directly stimulate colonic myenteric plexus -> increase motility + secretion
Bulk-forming	Psyllium/ispaghula, methylcellulose	Absorb water, increase stool bulk -> mechanical stimulation of peristalsis
Stool softener	Docusate	Surfactant - allows water/lipid penetration into stool

Lactulose - synthetic disaccharide, not absorbed/metabolised by human enzymes - colonic bacteria ferment it to short-chain fatty acids, which are the actual osmotically active/acidifying agents; also lowers colonic pH -> traps ammonia as NH4+, the basis for its separate use in hepatic encephalopathy
Macrogol (PEG) - purely osmotic, non-fermented, less gas/bloating than lactulose
Bulk-forming agents need adequate fluid intake - risk of obstruction if intake is inadequate

Hepatic encephalopathy - lactulose titrated to 2-3 soft stools/day; the mechanism is ammonia trapping, not just the laxative effect
Opioid-induced constipation - stimulant + osmotic combination typically needed; bulk-forming agents are relatively contraindicated (obstruction risk with reduced gut motility)
Bowel preparation for colonoscopy - high-dose macrogol (osmotic)

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