What is the recommended dosage and usage of Miralax (polyethylene glycol 3350) for treating constipation in pediatric patients?

MiraLAX (Polyethylene Glycol 3350) for Pediatric Constipation

For pediatric constipation, start MiraLAX at 0.8 g/kg/day (approximately 1 g/kg/day initially, then titrate down) mixed in at least 4-8 ounces of liquid, adjusting every 3 days to achieve 1-2 soft stools daily. 1, 2

Initial Dosing Strategy

• Start with 1 g/kg/day and adjust downward every 3 days based on response 2
• The effective maintenance dose averages 0.78-0.84 g/kg/day across multiple pediatric studies 3, 2
• For infants under 18 months, the same dosing applies (mean effective dose 0.78 g/kg/day) 3
• Even infants 0-5 months can be safely treated with this approach 3

Critical Administration Requirements

• Mix powder in minimum 4-8 ounces of liquid—insufficient volume is the most common cause of treatment failure 1
• Use juices containing sorbitol (apple, pear, prune) for synergistic osmotic effect 1
• Ensure adequate daily fluid intake beyond the mixing liquid, as PEG requires water to work osmotically 1, 4
• The medication can be mixed in water, juice, coffee, or tea 1

Before Starting Treatment: Rule Out Red Flags

• Check for fecal impaction on physical exam—may require manual disimpaction or enema before starting PEG 1
• Rule out bowel obstruction or paralytic ileus before initiating therapy 1
• Perform abdominal examination to assess for fecal retention 2

Expected Response Timeline and Adjustment

• Adjust dose every 3 days to achieve target of 1-2 soft stools daily 2
• Stool frequency should increase from baseline ~2 stools/week to ~17 stools/week within 8 weeks 2
• If no bowel movement after 3-4 days, add bisacodyl suppository (10mg) or glycerin suppository while continuing PEG 1, 4
• For persistent constipation despite optimization, add oral stimulant laxative (senna or bisacodyl) to the PEG regimen 1, 4

Efficacy Data Across Age Groups

• 97.6% response rate in infants under 18 months with mean treatment duration of 6.2 months 3
• 85% short-term (2 months) and 91% long-term (11 months) relief in children under 2 years 5
• In children with dysfunctional elimination, 18 became completely dry and 26 had decreased wetting when constipation was treated 6
• Soiling events decreased from 10/week to 1.3/week in children with encopresis 2

Long-Term Management

• Continue maintenance dosing once bowel movements normalize rather than stopping abruptly 1
• Response is durable over 6 months in adults, suggesting similar long-term efficacy in children 7, 1
• Mean treatment duration in pediatric studies ranges from 6-11 months 3, 5
• The effective dose often decreases over time (from 1.1 g/kg initially to 0.8 g/kg long-term) 5

Side Effects and Safety Profile

• Abdominal distension, flatulence, and nausea are generally mild 7, 1
• Transient diarrhea occurs in some patients but resolves with dose adjustment 3, 2
• Only 9 of 46 children in one study experienced diarrhea, all managed with dose reduction 6
• No significant adverse effects requiring discontinuation in pediatric studies 3, 2, 5
• Safe for use even in infants 0-5 months old 3

Common Clinical Pitfalls to Avoid

• Inadequate liquid volume (less than 4-8 ounces) significantly reduces efficacy 1, 4
• Not confirming adequate daily fluid intake beyond mixing liquid leads to treatment failure 1, 4
• Assuming treatment failure without optimizing dose and ensuring compliance for at least 2-4 weeks 1, 4
• Delaying rectal intervention beyond 3-4 days without bowel movement increases fecal impaction risk 1, 4
• Stopping therapy prematurely when symptoms improve rather than continuing maintenance 1
• Not checking for baseline fecal impaction before starting oral therapy 1

Special Populations

For children with dysfunctional elimination and voiding symptoms, treating constipation with PEG improves both bowel and bladder function—voided volume increases (146 to 210 mL) and post-void residual decreases significantly (92 to 48 mL) 6