What is the recommended starting dose of lactulose (osmotic laxative) for a 7-year-old child weighing 21.77 kg with severe persistent constipation who has not responded to Miralax (polyethylene glycol 3350)?

Lactulose Dosing for Severe Persistent Constipation in a 7-Year-Old

For a 21.77 kg child with severe persistent constipation unresponsive to Miralax, start lactulose at 40-60 mL/day (approximately 2-3 mL/kg/day) divided into 2-4 doses, with the goal of producing 2-3 soft stools daily. 1

Specific Dosing Protocol

Initial dosing for this 21.77 kg child should be 40-60 mL daily (approximately 1.8-2.8 mL/kg/day), divided into 2-4 doses throughout the day. 1 The FDA label states that for older children and adolescents, the total daily dose ranges from 40-90 mL, with the subjective goal being to produce 2-3 soft stools daily. 1

Short-Course Intensive Approach

Since you specifically want a short course for severe constipation, consider the higher end of dosing:

• Start with 60 mL/day divided into 3-4 doses (15-20 mL per dose) 1
• If inadequate response within 24-48 hours, increase to 80-90 mL/day divided into 3-4 doses 1
• The dose may be adjusted every day or two to produce 2-3 soft stools daily 1

Important Dosing Considerations

If the initial dose causes diarrhea, reduce the dose immediately and discontinue if diarrhea persists. 1 This is critical to avoid dehydration and electrolyte disturbances in children.

Evidence Comparison: Lactulose vs. Miralax

While you've noted Miralax failure, the evidence shows important differences:

• PEG 3350 (Miralax) demonstrates faster response times and superior efficacy compared to lactulose in multiple pediatric studies 2, 3
• One randomized trial showed PEG increased stool frequency from median 1 to 8 per week versus 1 to 7 with lactulose (p<0.01) 2
• However, lactulose remains effective for disimpaction when used at higher doses (4-6 mL/kg/day, maximum 120 mL/day), with 100% success rates by day 7 4

Alternative Approach for Severe Cases

Given the severity and Miralax failure, consider this escalation:

For disimpaction specifically, lactulose can be dosed at 4-6 mL/kg/day (87-130 mL/day for this child, maximum 120 mL/day) until resolution or up to 6 days. 4 This higher dosing regimen achieved successful disimpaction in all patients within 7 days in a controlled trial. 4

Practical Administration

• Divide the total daily dose into 3-4 administrations to improve tolerance 1
• Mix with water or juice to improve palatability 1
• Expect response within 24-48 hours, though it may take longer 1

Critical Pitfalls to Avoid

Do not use lactulose if there is concern for mechanical obstruction—perform physical exam and consider abdominal x-ray first. 5 Warning signs requiring further evaluation include abdominal distension, severe pain, or vomiting. 5

Monitor for excessive diarrhea, which can lead to dehydration and electrolyte abnormalities. 1 If diarrhea develops, reduce or stop lactulose immediately. 1

Adjunctive Therapy

Since this is severe persistent constipation:

• Add bisacodyl 10-15 mg daily with goal of one non-forced bowel movement every 1-2 days 5
• If impaction is present, use glycerin suppository or bisacodyl suppository (one rectally daily to twice daily) before starting oral therapy 5
• Ensure adequate fluid intake, as lactulose requires water to work effectively 5

When to Reassess

If no improvement after 48-72 hours at maximum tolerated dose, consider rectal examination to rule out fecal impaction. 5 If impaction is confirmed, manual disimpaction with appropriate pre-medication (analgesic ± anxiolytic) may be necessary before oral laxatives will be effective. 5

Once bowel movements normalize, transition to maintenance dosing (typically 40-60 mL/day) to prevent recurrence. 1