What is the appropriate first‑line regimen of oral polyethylene glycol 3350 (Miralax) for an adult with uncomplicated constipation?

Treatment of Constipation with MiraLAX (Polyethylene Glycol 3350)

For adults with uncomplicated chronic constipation, start with polyethylene glycol 3350 (MiraLAX) 17 grams once daily, mixed in 4-8 ounces of liquid, and continue for at least 1-2 weeks to achieve optimal effect. 1, 2, 3

Initial Dosing Regimen

• Standard dose is 17 grams once daily, dissolved in at least 4-8 ounces of any beverage including water, juice, soda, coffee, or tea 1, 2, 3
• Expect the first bowel movement within 2-4 days of starting therapy 2, 3
• Peak efficacy occurs during week 2 of treatment, not immediately, so patients must continue therapy even if initial response seems modest 4

Critical Implementation Points to Avoid Treatment Failure

Insufficient liquid volume is the most common cause of treatment failure 5, 2

• Mix the powder in a minimum of 4 ounces, but preferably 8 ounces of liquid 2, 3
• Patients must maintain adequate daily fluid intake throughout the day, beyond just the mixing liquid, because PEG requires water to work osmotically 5, 2
• Using juices with sorbitol content provides a synergistic osmotic effect 5

Expected Efficacy

Based on moderate-certainty evidence from the American Gastroenterological Association 1, 2:

• Increases complete spontaneous bowel movements by 2.90 per week 2
• Increases total spontaneous bowel movements by 2.30 per week 2
• 312 more patients per 1,000 meet responder criteria compared to placebo 2
• 454 more patients per 1,000 report global symptom relief 2
• Treatment success (defined as relief of constipation criteria for ≥50% of treatment weeks) occurs in 52% of patients versus 11% with placebo 6

Duration of Treatment

Do not stop therapy prematurely 7, 3, 8:

• FDA labeling states therapy is intended for up to 2 weeks for occasional constipation 3
• However, response is durable over 6 months in clinical trials, and the American Gastroenterological Association supports continued maintenance therapy 1, 2, 6
• 61.7% of patients require additional laxative interventions within 30 days of stopping, indicating many patients benefit from ongoing maintenance rather than short-term use 2, 8
• Long-term use up to 6-12 months has been demonstrated safe and effective 6, 9

Dose Titration

• There is no clear maximum dose 1
• Titrate based on symptom response and side effects 1
• If 17 grams daily is insufficient after 2 weeks of optimal dosing with adequate hydration, the dose can be increased 1
• Higher single doses (68 grams) have been studied for rapid relief within 24 hours, producing an average of 2.2 bowel movements in 24 hours without adverse effects 4

Managing Inadequate Response

If constipation persists after 3-4 days despite optimal PEG dosing 5, 2:

• Add a stimulant laxative (senna 8.6-17.2 mg daily or bisacodyl 5-10 mg daily) to the PEG regimen, as they work through complementary mechanisms—PEG softens stool osmotically while stimulants enhance colonic motility 1, 2
• Consider adding a bisacodyl suppository (10 mg) or glycerin suppository while continuing oral PEG 5
• Rule out fecal impaction, which may require manual disimpaction or enema before continuing oral therapy 5

Safety Profile and Side Effects

Common adverse effects include 1, 2, 3:

• Abdominal distension, bloating, cramping, and flatulence 1, 2
• Nausea (occurs in some patients) 2, 3, 9
• Diarrhea occurs in 158 more patients per 1,000 compared to placebo 2
• Most adverse events are mild to moderate and consistent with expected laxative effects 2, 9

Contraindications and precautions 3:

• Do not use if bowel obstruction or paralytic ileus is suspected (symptoms include nausea, vomiting, abdominal pain or distention) 5, 3
• Discontinue if allergic reaction (hives, skin rash) occurs 3
• Use with caution in pregnancy unless prescribed by a physician 3

Long-Term Safety

• No significant changes in electrolytes, calcium, glucose, BUN, creatinine, or serum osmolality occur with chronic use 4, 6
• Monthly laboratory monitoring in 6-month studies showed no clinically significant abnormalities in hematology, chemistry panels, or urinalysis 6
• Treatment up to 52 weeks demonstrates sustained efficacy with continued tolerability 9

Common Clinical Pitfalls

• Assuming treatment failure without first optimizing dose and ensuring adequate fluid intake leads to premature discontinuation 5
• Stopping therapy too early (before 2 weeks or as soon as symptoms improve) often leads to relapse 7
• Not confirming that patients are mixing the powder in sufficient liquid volume 5, 2
• Failing to counsel patients that peak efficacy occurs in week 2, not immediately 4