MiraLAX Dosing for a 9-Month-Old Infant with Constipation
For a 9-month-old infant with constipation, polyethylene glycol 3350 (MiraLAX) should be dosed at 0.8-1 g/kg/day, which typically translates to approximately 7-9 grams daily for an average-sized 9-month-old, dissolved in 2-4 ounces of any beverage. 1
Age-Appropriate First-Line Therapy
- Polyethylene glycol (PEG) 3350 is the laxative of first choice for infants 6 months and older according to the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition 1
- For infants under 6 months, lactulose or lactitol-based medications are the recommended alternatives 2
- At 9 months of age, this infant falls squarely within the appropriate age range for PEG 3350 therapy 1, 3
Specific Dosing Instructions
Initial dosing:
- Start with 0.8-1 g/kg/day as a single daily dose 1
- For a typical 9-month-old weighing 8-9 kg, this equals approximately 7-9 grams daily 1
- Dissolve the powder completely in 2-4 ounces of any beverage (breast milk, formula, water, or juice) 1, 4
Titration strategy:
- The goal is to produce 2-3 soft, painless stools daily 1
- Adjust the dose based on stool frequency and consistency 1
- PEG is highly effective with dose-dependent efficacy 5
Important Clinical Considerations
Before initiating therapy:
- Rule out fecal impaction through digital rectal examination if the infant appears to have significant stool burden 1, 6
- If impaction is present, consider a glycerin suppository (pediatric formulation) first for mechanical disimpaction before starting maintenance PEG therapy 1, 6
Maintenance therapy duration:
- Treatment typically needs to continue for many months before the child regains normal bowel motility and rectal perception 1
- Premature discontinuation is a common pitfall—parents often cease treatment too soon 1
Adjunctive Non-Pharmacological Measures
- Increase fluid intake to maintain proper hydration 1
- Offer fruit juices containing sorbitol (prune, pear, apple) which can help increase stool frequency 1
- If the infant is eating solid foods, emphasize high-fiber options like fruits, vegetables, and whole grains 1
Safety Profile
- PEG 3350 has an excellent safety profile with only minor adverse events reported 5
- Common side effects include bloating, abdominal discomfort, and flatulence 4
- Ensure adequate fluid intake throughout the day 4
- In patients predisposed to water and electrolyte imbalances, monitoring of serum electrolytes should be considered 5
Common Pitfalls to Avoid
- Do not rely solely on dietary changes if impaction is present, as this can worsen constipation 1
- Do not use stimulant laxatives (bisacodyl, senna) as first-line therapy in infants—these are not appropriate for this age group and safer alternatives exist 1
- Do not discontinue therapy prematurely—maintenance may need to continue for months 1
- Ensure the powder is fully dissolved before administration—do not give if clumps remain 7
FDA Labeling Note
The FDA-approved labeling for over-the-counter PEG 3350 states "children 16 years of age or under: ask a doctor," which means this requires physician prescription and supervision for a 9-month-old 7. However, extensive clinical evidence and pediatric gastroenterology guidelines strongly support its use in this age group under medical guidance 1, 2, 3, 5.