What is the appropriate use of Miralax (polyethylene glycol) for constipation?

Miralax (Polyethylene Glycol) for Constipation

Miralax (PEG 3350) is the most strongly recommended first-line pharmacologic treatment for chronic constipation, with moderate-quality evidence showing superior efficacy over placebo and other laxatives. 1

Dosing and Administration

• Standard dose: 17 g mixed in 8 ounces of liquid once daily 1
• FDA-approved for occasional constipation, but proven effective for chronic idiopathic constipation (CIC) for up to 6-12 months 1, 2
• Generally produces a bowel movement within 1-3 days 3
• For faster relief, higher doses (68 g) can provide effective relief within 24 hours, though this is off-label 4

Efficacy Data

PEG demonstrates robust clinical benefits with moderate certainty of evidence:

• Increases complete spontaneous bowel movements (CSBMs) by 2.90 per week compared to placebo (95% CI 2.12-3.68) 1
• Increases spontaneous bowel movements (SBMs) by 2.30 per week (95% CI 1.55-3.06) 1
• Response rate 3.13 times higher than placebo (RR 3.13, CI 2.00-4.89), translating to 312 more responders per 1,000 patients 1
• 454 more patients per 1,000 achieve global symptom relief compared to placebo 1
• Improves stool form, straining, and overall satisfaction 1
• Does not significantly improve abdominal pain 1

Safety Profile

PEG has an excellent safety profile with no clinically significant adverse effects:

• No differences in serious adverse events compared to placebo (though confidence intervals are wide due to small event numbers) 1
• Common mild-to-moderate side effects: bloating, flatulence, diarrhea (158 more per 1,000 compared to placebo) 1
• No clinically significant changes in electrolytes, calcium, glucose, BUN, creatinine, or serum osmolality even with chronic use up to 12 months 5, 2
• No evidence of tachyphylaxis with long-term use 2
• Safe in elderly patients (age 65+) 2

Treatment Algorithm Position

The 2023 AGA-ACG guidelines position PEG early in the treatment algorithm:

1. Consider fiber supplementation first (particularly psyllium) for patients with low dietary fiber intake 1, 6
2. Use PEG as first-line pharmacologic therapy, either after a trial of fiber or in combination with fiber 1
3. PEG is preferred over magnesium oxide, which has only conditional recommendation with very low certainty evidence 1

Comparative Effectiveness

• Superior or equal efficacy to lactulose, tegaserod, and prucalopride in head-to-head trials 1, 7
• More effective than placebo and lactulose as an osmotic laxative 7
• As effective as enemas for fecal impaction while avoiding hospitalization 7

Special Populations

• Medication-induced constipation: 78.3% treatment success rate vs. 39.1% with placebo (P < 0.001) 5
• Opioid-induced constipation: Use PEG, but if standard laxatives fail, consider methylnaltrexone (0.15 mg/kg subcutaneously every other day), except in post-operative ileus or mechanical obstruction 1
• Elderly patients: Equally safe and effective as in younger adults 5, 2

Key Clinical Pearls

• PEG is widely available over-the-counter and relatively inexpensive, making it highly accessible 1
• Ensure adequate fluid intake (8-10 ounces of water with each dose) 6
• Before escalating therapy, rule out: fecal impaction, bowel obstruction, hypercalcemia, hypokalemia, hypothyroidism, diabetes, and constipating medications 1, 6
• Response is typically best by week 2 of treatment and remains durable over time 4, 2
• No incontinence, severe cramping, or diarrhea reported even at higher doses 4