What is the weight‑based dosing of MiraLAX (polyethylene glycol 3350) for pediatric patients?

MiraLAX Weight-Based Dosing for Pediatric Patients

For pediatric functional constipation, initiate PEG 3350 at 0.4 g/kg/day as the starting dose for children 2 years and older, with a mean effective maintenance dose of approximately 0.78 g/kg/day for infants under 18 months. 12

Dosing by Age Group

Infants < 18 Months

• Starting dose: 0.88 g/kg/day (range 0.26–2.14 g/kg/day) 2
• Effective maintenance dose: 0.78 g/kg/day (range 0.26–1.26 g/kg/day) 2
• Success rate: 97.6% achieved relief of constipation at these doses 2
• Duration studied: Mean 6.2 months (range 3 weeks to 21 months) 2

Children 2–12 Years

• Recommended starting dose: 0.4 g/kg/day 1
• Alternative dosing range: 1.0–1.5 g/kg/day studied with 95.3% success rate 3
• Rationale: The 0.4 g/kg dose demonstrated 74% treatment success (≥3 bowel movements per week) versus 42% with placebo, with significantly improved stool consistency and less adverse effects compared to higher doses 1

Fecal Impaction (Disimpaction Protocol)

• Effective doses: 1.0–1.5 g/kg/day for 3 days 4
• Success rate: 95% disimpaction achieved at these higher doses versus 55% at lower doses (0.25–0.5 g/kg/day) 4
• Bowel movement frequency: Increased from baseline 1.4 movements to 12.3 movements over 5 days at 1.5 g/kg/day 4

Practical Measurement Guidance

Common pitfall: Standard household measuring devices vary in accuracy, which can affect dosing consistency. 5

• 1 leveled teaspoon ≈ 3.3 grams of PEG 3350 5
• 1 leveled tablespoon ≈ 10 grams of PEG 3350 5
• 1 capful (manufacturer) = 17 grams (adult dose) 66
• Best practice: Use one dedicated measuring device and level the dose to improve consistency, particularly for patients sensitive to dose variability 5

Dosing Algorithm

1. For maintenance therapy (chronic constipation):

   • Infants < 18 months: Start 0.78 g/kg/day 2
   • Children ≥ 2 years: Start 0.4 g/kg/day 1
   • Titrate upward based on response 7

2. For fecal impaction:

   • Use 1.0–1.5 g/kg/day for 3 days 4
   • Then transition to maintenance dosing

3. Dose adjustment:

   • If inadequate response: Increase dose gradually 7
   • If diarrhea develops: Reduce dose (transient diarrhea resolves with adjustment) 2

Safety Profile in Pediatrics

• Common adverse effects: Transient diarrhea (13%), bloating (18%), nausea (5%), vomiting (5%), cramping (5%) 4
• Dose-related effects: Diarrhea and bloating more prevalent at higher doses (1.0–1.5 g/kg/day) 4
• Electrolyte safety: No clinically significant electrolyte changes noted in pediatric studies 4
• Duration of safe use: Up to 21 months documented in infants, with mean 6.2 months 2

Important Caveats

Contamination risk: One case report documented suspected ethylene glycol contamination of generic PEG 3350 causing acute kidney injury with calcium oxalate crystalluria. 8 While extremely rare, monitor for unexplained high anion gap metabolic acidosis, elevated osmolar gap, or AKI in patients on PEG 3350. 8

Evidence quality note: The systematic review by Michaud et al. concluded that while PEG is safe in children < 24 months, "evidence to establish appropriate dosage regimens does not exist" due to variable dosing categories across studies. 9 However, the prospective randomized trial by Michaud et al. (2008) provides the strongest evidence for the 0.4 g/kg/day starting dose in children ≥ 2 years. 1

First-line status: The American Gastroenterological Association recommends PEG laxatives as first-line pharmacologic treatment for chronic constipation, though this recommendation is based on low-quality evidence for specific populations. 7