What is the appropriate lactulose dose for constipation in an 8.5‑kg child?

Lactulose Dosing for an 8.5 kg Infant

For an 8.5 kg infant with constipation, start with lactulose 2.5 to 10 mL daily in divided doses, adjusting to produce 2-3 soft stools per day. 1

Initial Dosing Strategy

• Begin with 5 mL daily (split into 2.5 mL twice daily) for an 8.5 kg infant, as the FDA label recommends 2.5 to 10 mL daily in divided doses for infants 1
• The goal is to produce 2-3 soft stools daily, not diarrhea 1
• If the initial dose causes diarrhea, reduce the dose immediately and discontinue if diarrhea persists 1

Dose Titration

• Adjust the dose every 1-2 days based on stool frequency and consistency 1
• Increase gradually by 2.5 mL increments if constipation persists after 2-3 days 1
• Maximum dose for infants is typically 10 mL daily, though this should be divided into 2-3 doses throughout the day 1

Pre-Treatment Assessment

Before starting lactulose, you must rule out more serious conditions:

• Perform a digital rectal examination to distinguish true fecal impaction from simple constipation 2
• Rule out mechanical bowel obstruction through physical examination and consider abdominal imaging if symptoms are severe 2, 3
• Do not use lactulose if the infant has undiagnosed acute abdominal pain, as this is an absolute contraindication 2

Special Considerations for Impaction

• If true fecal impaction is present, lactulose alone may be inadequate for initial disimpaction in an infant 2
• Consider a pediatric glycerin suppository first to mechanically disrupt the impacted stool mass before starting lactulose 2
• Once disimpaction is achieved, transition to maintenance lactulose therapy 2

Practical Administration Tips

• Lactulose can be mixed with water, milk, or fruit juice to improve palatability 1
• Administer doses at consistent times each day (e.g., morning and evening) 1
• Increase fluid intake to enhance the osmotic effect of lactulose 2

Monitoring and Follow-Up

• Reassess within 24-48 hours to determine response to therapy 3
• Improvement may occur within 24 hours but may take up to 48 hours or longer 1
• If no improvement after 3 days at maximum tolerated dose, consider adding polyethylene glycol (PEG) as it is more effective than lactulose for chronic constipation 4

Common Pitfalls to Avoid

• Failing to reduce dose when diarrhea occurs leads to dehydration risk in infants 1
• Starting with too high a dose (>5 mL daily) increases the risk of cramping, bloating, and diarrhea 1
• Not addressing underlying causes such as inadequate fluid intake or dietary factors 2
• Using lactulose for impaction without first achieving mechanical disimpaction 2

Alternative Considerations

While lactulose is FDA-approved and commonly used, polyethylene glycol (PEG) demonstrates superior efficacy in terms of stool frequency, consistency, and relief of abdominal pain compared to lactulose in both adults and children 4. However, lactulose has prebiotic effects that may benefit the colonic microbiome by increasing bifidobacteria 5, which PEG does not provide 5.