Bulk-Forming Agents (Fiber Supplements)
Mechanism: Increase stool bulk and water content → stimulate peristalsis
Onset: 12–72 hours
- Examples:
- Psyllium (Metamucil)
- Methylcellulose (Citrucel)
- Calcium polycarbophil (FiberCon)
- Clinical Notes:
- First-line for many patients
- Requires adequate hydration
- May cause bloating or gas
Osmotic Laxatives
Mechanism: Draw water into colon → soften stool and increase motility
Onset: 0.5–6 hours (saline), 24–72 hours (others)
A. Saline Osmotics
- Examples:
- Magnesium hydroxide (Milk of Magnesia)
- Magnesium citrate
- Sodium phosphate (Fleet enema)
- Cautions: Electrolyte disturbances, renal insufficiency
B. Non-saline Osmotics
- Examples:
- Polyethylene glycol (PEG) (MiraLAX, GoLYTELY)
- Lactulose (also for hepatic encephalopathy)
- Sorbitol
- Clinical Notes:
- PEG preferred for chronic use
- Lactulose can cause bloating, gas
Stimulant Laxatives
Mechanism: Stimulate enteric nerves and colonic contraction
Onset: 6–12 hours oral; faster rectally
- Examples:
- Senna (Senokot)
- Bisacodyl (Dulcolax, oral or suppository)
- Castor oil (rarely used)
- Clinical Notes:
- Risk of cramping and dependency with chronic use
- Good for opioid-induced constipation (with stool softeners)
Stool Softeners (Emollients)
Mechanism: Reduce surface tension → allow water/fat to enter stool
Onset: 12–72 hours
- Examples:
- Docusate sodium (Colace)
- Docusate calcium
- Clinical Notes:
- Often used in post-op or post-partum patients
- Minimal efficacy in severe constipation
Lubricants
Mechanism: Coat stool to prevent water absorption
Onset: 6–8 hours oral
- Examples:
- Mineral oil
- Clinical Notes:
- Risk of aspiration pneumonia (avoid in elderly/bedridden)
- Not for long-term use