MiraLAX (Polyethylene Glycol 3350) Mechanism of Action
Polyethylene glycol 3350 works as an osmotic laxative by retaining water in the stool through its long-chain polymer structure, which softens stool and increases bowel movement frequency. 1, 2
Mechanism of Action
PEG 3350 is a long-chain polymer of ethylene oxide that acts osmotically by binding water molecules in the intestinal lumen. 1
The medication retains water in the stool, creating a softer consistency and increased stool volume that stimulates bowel movements. 2
Unlike electrolyte-containing laxatives or secretagogues, PEG 3350 does not stimulate colonic contractions or activate ion channels—it works purely through physical water retention. 1
The osmotic effect requires adequate fluid intake beyond just the mixing liquid for the medication to work effectively. 3, 4
Recommended Dosing
Standard adult dose: 17 grams once daily, mixed in 4–8 ounces of liquid (water, juice, soda, coffee, or tea). 1, 3, 2
First bowel movement typically occurs within 2–4 days, with optimal efficacy achieved after 1–2 weeks of continuous use. 3, 2
If no response after 2 weeks of optimal dosing with adequate hydration, the dose may be increased (e.g., to 34 grams daily) based on individual response. 3
For persistent constipation after 3–4 days, add a stimulant laxative (senna 8.6–17.2 mg daily or bisacodyl 5–10 mg daily) to exploit complementary mechanisms. 3, 4
Clinical Efficacy
Increases complete spontaneous bowel movements by approximately 2.9 per week compared with placebo. 1, 3
Increases total spontaneous bowel movements by approximately 2.3 per week versus placebo. 1, 3
52% of patients achieve treatment success (relief of constipation criteria for ≥50% of treatment weeks) versus 11% with placebo. 3, 5
Response is durable for up to 6 months, supporting long-term maintenance therapy when needed. 1, 3, 5
Safety Profile
Most common adverse effects: abdominal distension, bloating, cramping, flatulence, and nausea—typically mild to moderate. 1, 3, 2
Diarrhea occurs in 158 additional patients per 1,000 compared with placebo. 3
No clinically significant changes in electrolytes, calcium, glucose, BUN, creatinine, or serum osmolality have been observed with chronic use. 3, 6, 5
Long-term use up to 52 weeks maintains efficacy and tolerability without abnormal laboratory findings. 3, 5
Contraindications and Precautions
Do not use in patients with bowel obstruction, paralytic ileus, or symptoms of nausea, vomiting, or abdominal distention that may indicate obstruction. 4, 2
Check for fecal impaction before initiating therapy, which may require manual disimpaction or enema first. 7, 4
Not FDA-approved for children, though widely used off-label in pediatric populations with evidence supporting efficacy. 2, 8
Use in pregnancy only if prescribed by a physician. 2
Common Clinical Pitfalls
Insufficient liquid volume (less than 4 ounces) is a common cause of treatment failure—patients must use at least 4 ounces, preferably 8 ounces, for adequate osmotic activity. 3, 4
Inadequate daily fluid intake throughout the day (beyond just the mixing liquid) reduces efficacy significantly. 3, 7
Assuming treatment failure without first optimizing dose and confirming compliance leads to premature discontinuation. 7
Not adding rectal therapy (bisacodyl or glycerin suppository) after 3–4 days without bowel movement increases risk of fecal impaction. 7, 4