What is the difference between Psyllium and Miralax (Polyethylene Glycol) for treating constipation?

Psyllium vs. Miralax for Constipation

For treating constipation, polyethylene glycol (Miralax) is recommended over psyllium due to its stronger evidence for efficacy, faster onset of action, and better clinical outcomes. 1

Mechanism of Action

Psyllium

• Type: Soluble fiber (bulk-forming laxative)
• Mechanism: Increases stool weight by absorbing water and forming a gel-like substance that adds bulk
• Secondary effects: May have prebiotic potential, stimulating beneficial gut bacteria 1
• Time to effect: Generally produces bowel movement in 12-72 hours 2

Polyethylene Glycol (Miralax)

• Type: Osmotic laxative
• Mechanism: Forms an isotonic solution that prevents water absorption in the intestines
• Effect: Increases intestinal volume, softens stool, and facilitates evacuation
• Time to effect: Generally produces a bowel movement in 1-3 days 3

Efficacy Comparison

Psyllium

• May increase spontaneous bowel movements (SBMs) by 2.32 per week (low certainty evidence) 1
• Improves global relief symptoms with relative risk of 1.86 compared to placebo 1
• Limited effect on stool consistency 1
• No data on complete spontaneous bowel movements (CSBMs) 1

Polyethylene Glycol (PEG)

• Increases CSBMs by 2.90 per week (moderate certainty evidence) 1
• Increases SBMs by 2.30 per week 1
• Higher responder rate compared to placebo 1
• Recommended by the American Gastroenterological Association with a strong recommendation (moderate certainty evidence) 1

Clinical Application Algorithm

1. For mild constipation or as initial therapy:

   • Consider psyllium if patient has adequate fluid intake
   • Dosage: 5g twice daily or 3.6g three times daily 1
   • Must be taken with 8-10 ounces of fluid 1

2. For moderate to severe constipation:

   • PEG is preferred (17g daily mixed in 8 ounces of liquid) 1
   • Response has been shown to be durable over 6 months 1

3. For rapid relief:

   • PEG at higher doses (68g) can provide relief within 24 hours 4
   • Average time to first bowel movement: 14.8 hours vs. 27.3 hours for placebo 4

4. For opioid-induced constipation:

   • Start with PEG 1
   • If inadequate response, consider adding peripherally acting μ-opioid receptor antagonists like methylnaltrexone 1

Side Effects and Tolerability

Psyllium

• Generally well-tolerated
• Main side effect: flatulence 1
• Rare: withdrawal due to diarrhea (very low certainty evidence) 1
• Must be taken with adequate fluid to prevent esophageal obstruction

PEG

• Well-tolerated with minimal systemic absorption
• Side effects: abdominal distension, loose stool, flatulence, and nausea 1
• No significant electrolyte disturbances at standard doses 4

Special Considerations

1. Fluid intake:

   • Psyllium requires adequate fluid intake to be effective 1
   • Those in the lowest quartile of fluid intake are more likely to be constipated 1

2. Combination therapy:

   • A trial of fiber supplement can be considered before PEG use or in combination with PEG 1
   • Psyllium plus senna may be more effective than psyllium alone for severe constipation 5

3. Onset of action:

   • PEG works faster (1-3 days) 3 compared to psyllium (12-72 hours) 2

Common Pitfalls to Avoid

1. Inadequate fluid intake with psyllium: Can worsen constipation or cause esophageal obstruction
2. Overreliance on fiber alone for severe constipation: May delay effective treatment
3. Ignoring underlying causes: Rule out impaction, obstruction, hypercalcemia, hypokalemia, hypothyroidism, and diabetes mellitus 1
4. Continuing constipating medications: Discontinue nonessential constipating medications when possible 1

In summary, while both agents are effective for constipation, PEG (Miralax) has stronger evidence supporting its efficacy, works faster, and is recommended with a strong recommendation by current guidelines. Psyllium may be appropriate for mild constipation or as an adjunct to PEG therapy.