Tea h me laxative puragativevin very simple manner
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Laxatives and Purgatives - Simple Guide
What are They?
A laxative promotes a soft/formed bowel movement (gentle, takes hours to days).
A purgative (or cathartic) produces a strong, watery evacuation (fast, within 1-3 hours).
They are essentially the same class of drugs - purgative just means a stronger/faster laxative.
Classification (By How They Work)
1. Bulk-Forming Laxatives
Onset: 1-3 days (slow)
Examples: Psyllium (Isabgol), Bran, Methylcellulose, Calcium polycarbophil
How they work:
- These are dietary fibers that absorb water in the intestine
- They swell up and increase the bulk/size of the stool
- This stretches the bowel wall and triggers the urge to defecate
- Think of them like a sponge - they soak up water and make stool soft and bulky
Key point: Must drink plenty of water with these. Safest type - can be used long term.
2. Osmotic Laxatives
Onset: 1-3 hours (fast) = Purgative effect
Examples:
- Magnesium sulfate (Epsom salt)
- Milk of Magnesia (Magnesium hydroxide)
- Magnesium citrate
- Polyethylene Glycol (PEG/Macrogol)
- Lactulose, Sorbitol
How they work:
- These hold water inside the intestine by osmosis
- Water cannot be absorbed, so stool stays liquid
- The extra fluid stretches the bowel and causes rapid emptying
- Magnesium also stimulates CCK (a gut hormone), increasing intestinal movement
Key point: Magnesium-based ones can cause hypermagnesemia in patients with kidney disease - use with caution!
3. Stimulant (Irritant) Laxatives
Onset: 6-8 hours
Examples:
- Bisacodyl (Dulcolax)
- Senna (Senokot)
- Cascara sagrada
- Castor oil
How they work:
- They directly irritate/stimulate the nerves in the intestinal wall
- This increases gut muscle contractions (peristalsis)
- Senna and cascara are anthraquinone derivatives - they are converted to active form by gut bacteria in the colon
- Bisacodyl is a diphenylmethane derivative - also reduces water absorption from colon
Key point: Short-term use is fine. Long-term use can lead to laxative dependence and a "lazy bowel."
4. Stool Softeners (Surfactant Laxatives)
Onset: 1-3 days
Examples: Docusate sodium (Colace)
How they work:
- They act like a detergent/soap
- They lower the surface tension of the stool
- This allows water and fat to mix into the stool, softening it
- No real stimulation of the gut - just makes stool easier to pass
Key point: Weakest type. Good after surgery or childbirth when straining must be avoided.
5. Lubricant Laxatives
Examples: Liquid paraffin (mineral oil), Glycerin suppository
How they work:
- They coat the stool and bowel wall with an oily film
- Stool slides out easily
- Glycerin suppository also draws water into the rectum (osmotic)
Key point: Liquid paraffin can interfere with absorption of fat-soluble vitamins (A, D, E, K) if used long term.
Quick Comparison Table
| Type | Example | Onset | Mechanism |
|---|---|---|---|
| Bulk-forming | Isabgol, Bran | 1-3 days | Absorbs water, swells stool |
| Osmotic | Lactulose, MgSO4, PEG | 1-3 hrs (purgative) | Retains water in gut by osmosis |
| Stimulant | Bisacodyl, Senna | 6-8 hrs | Irritates gut, increases peristalsis |
| Stool softener | Docusate | 1-3 days | Detergent action, softens stool |
| Lubricant | Liquid paraffin | Variable | Coats stool, eases passage |
Laxative vs Purgative - Simple Difference
| Laxative | Purgative | |
|---|---|---|
| Effect | Soft, formed stool | Watery, explosive evacuation |
| Speed | Slow (days) | Fast (1-3 hours) |
| Strength | Mild | Strong |
| Example | Bulk-forming agents | High-dose saline (MgSO4) |
A purgative is just a laxative given in a larger dose or using a more powerful agent - same drugs, higher dose = purgative effect.
Clinical Uses (When to Use Which)
- Constipation (routine): Bulk-forming first, then osmotic (lactulose/PEG)
- Pre-colonoscopy bowel prep: PEG solution (4 litres)
- Hepatic encephalopathy: Lactulose (traps ammonia in gut as NH4+)
- Opioid-induced constipation: Stimulant (senna) + osmotic
- Post-op/post-delivery (avoid straining): Docusate (stool softener)
- Quick purgation needed: Magnesium sulfate or castor oil
Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics, Chapter 54; Katzung's Basic and Clinical Pharmacology, 16th Edition
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