Lactulose Syrup Dosing for Constipation
For chronic idiopathic constipation, start lactulose at 15-30 mL (10-20 g) once daily, preferably in the evening, and titrate up to a maximum of 60 mL (40 g) daily if needed to produce 2-3 soft stools per day. 1, 2
FDA-Approved Dosing
The FDA label specifies the standard adult oral dosage as 2-3 tablespoonfuls (30-45 mL, containing 20-30 g of lactulose) three or four times daily, adjusted to produce 2-3 soft stools daily. 2 However, this higher dosing is primarily for hepatic encephalopathy rather than simple constipation.
Guideline-Based Dosing for Constipation
The American Gastroenterological Association and American College of Gastroenterology recommend a more conservative approach for chronic constipation:
- Initial dose: 10-20 g (15-30 mL or 1-2 packets) daily 1
- Maximum dose: 40 g (60 mL or 2-4 packets) daily if needed 1
- Timing: Single evening dose to optimize compliance and minimize side effects 3
Dosing Strategy and Titration
Start low and titrate based on response: 3
- Begin with 15-30 mL once daily in the evening 4
- Titrate every few days based on stool frequency and consistency 4
- Target 2-3 soft stools per day 2
- If inadequate response after 2-4 weeks at maximum tolerated dose (up to 60 mL daily), consider adding or switching to alternative agents 4
Position in Treatment Algorithm
Lactulose is recommended as a second-line option for adults with chronic idiopathic constipation who fail or are intolerant to over-the-counter therapies (fiber, polyethylene glycol, magnesium oxide). 1, 3 This is a conditional recommendation with very low certainty of evidence. 1
Common Pitfalls and Side Effect Management
Bloating and flatulence are dose-dependent and occur in approximately 20% of patients: 3, 5
- These gastrointestinal side effects may limit clinical use 1
- Starting at lower doses (10-20 g daily) and titrating upward minimizes these symptoms 3, 4
- If bloating is intolerable despite dose adjustment, consider switching to alternative laxatives 3
Excessive dosing can lead to diarrhea with potential complications including hypokalemia and hypernatremia. 3
Evidence Quality Considerations
The evidence supporting lactulose for constipation is notably weak: 3
- Only two small RCTs from over 40 years ago in elderly nursing home populations form the basis of guideline recommendations 1, 3
- Long-term safety data is lacking 3
- Despite weak evidence, lactulose remains widely used due to availability and low cost 1
More recent Japanese data suggests optimal dosing at 26 g/day with good tolerability up to 39 g/day. 6