MiraLAX Dosing for an 8-Month-Old Infant
For an 8-month-old infant with constipation, the recommended dose of polyethylene glycol 3350 (MiraLAX) is 0.78 g/kg/day, mixed in 4-8 ounces of liquid. 1
Age-Specific Dosing Evidence
Infants 6-11 months old require a mean effective maintenance dose of 0.78 g/kg/day (range 0.26-1.26 g/kg/day), based on a study of 28 infants under 18 months where PEG relieved constipation in 97.6% of patients 1
The initial starting dose can range from 0.26-2.14 g/kg/day, with adjustments made based on response 1
For an average 8-month-old weighing approximately 8-9 kg, this translates to roughly 6-7 grams daily (about 1/3 of an adult packet) 1
Critical Administration Guidelines
Mix the powder in at least 4-8 ounces of liquid - insufficient liquid volume is the most common cause of treatment failure 2
Acceptable mixing liquids include:
- Water, juice, formula, or breast milk 2
- Juices containing sorbitol (like prune or pear juice) provide synergistic osmotic effect 2
- Ensure powder is fully dissolved before administration - do not give if clumps remain 3
Important Safety Considerations
Before initiating therapy:
- Rule out bowel obstruction or paralytic ileus 2
- Check for fecal impaction, which may require manual disimpaction or enema first 2
- Ensure adequate daily fluid intake beyond just the mixing liquid, as PEG requires water to work osmotically 2
Expected side effects (generally mild):
- Transient diarrhea in 4 of 28 infants, resolved with dose adjustment 1
- Increased gas per rectum (1 infant) 1
- Abdominal distension, flatulence, and nausea may occur 4
Treatment Duration and Monitoring
- Mean duration of therapy in infants was 6.2 months (range 3 weeks to 21 months) 1
- Continue maintenance dosing once bowel movements normalize rather than stopping abruptly 2
- Response to treatment is durable over 6 months 2
When Initial Treatment Fails
If no bowel movement after 3-4 days:
- Add glycerin suppository while continuing PEG 2
- For persistent constipation, consider adding oral stimulant laxative (senna) to PEG regimen 2
If treatment appears ineffective, first optimize:
- Verify adequate liquid volume for mixing 2
- Confirm adequate daily fluid intake separate from mixing liquid 2
- Ensure compliance with daily dosing 2
- Consider dose adjustment upward within the safe range 1
Key Clinical Pitfalls to Avoid
- Inadequate liquid volume reduces efficacy significantly 2
- Not confirming adequate fluid intake beyond the mixing liquid leads to treatment failure 2
- Assuming treatment failure without first optimizing dose and ensuring compliance 2
- Using the FDA-labeled adult dose (17g) without weight-based adjustment for infants 3, 1
Regulatory Context
Note that the FDA label states "children 16 years of age or under: ask a doctor" 3, but polyethylene glycol is authorized and effective for infants over 6 months based on pediatric gastroenterology evidence 5, and the American Academy of Pediatrics recommends PEG as the most effective treatment for functional constipation in children 6