What is the recommended dose of lactulose for treating constipation?

Lactulose Dosing for Constipation

Start lactulose at 15 g (approximately 15-30 mL) once daily, preferably in the evening, and titrate upward based on symptom response to a maximum of 40 g (60 mL) daily, with a goal of producing 2-3 soft, non-forced bowel movements per day. 1

Initial Dosing Strategy

• Begin with 15 g daily (10-20 g range acceptable), which corresponds to 15-30 mL of lactulose solution, taken as a single evening dose to optimize compliance and minimize dose-dependent gastrointestinal side effects 1, 2
• The FDA label indicates the usual adult oral dosage for constipation is 30-45 mL (20-30 g) three or four times daily, but current guideline recommendations favor starting lower to reduce side effects 3
• For postpartum constipation specifically, research supports starting at 15 mL once daily with maintenance dosing of 5-15 mL based on response 4

Dose Titration Algorithm

• Increase the dose every few days based on stool frequency and consistency, titrating per symptom response and tolerability 1
• The target is 2-3 soft, non-forced bowel movements daily 2, 5
• Maximum recommended dose is 40 g (60 mL) daily according to the American Gastroenterological Association guidelines 2, 5
• If inadequate response after 2-4 weeks at maximum tolerated dose, or if bloating becomes intolerable, consider adding short-term senna as rescue therapy or switching to alternative agents 6

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) 2, 5
• This is a conditional recommendation with very low certainty of evidence, based primarily on two small RCTs from over 40 years ago in elderly nursing home populations 2
• Despite weak evidence, lactulose remains the only osmotic agent studied in pregnancy, making it a preferred option in this population 1

Critical Side Effects and Management

• Bloating and flatulence occur in approximately 20% of patients and are dose-dependent, which may limit clinical use 2, 5
• These gastrointestinal side effects can be minimized by starting at lower doses (10-20 g daily) and titrating slowly 2
• Excessive dosing can cause diarrhea with hypokalemia and hypernatremia, particularly in elderly patients, those on diuretics, or patients with hepatic encephalopathy receiving high doses 2
• Monitor baseline serum potassium before initiating therapy and during dose titration, especially in high-risk populations 2

Special Clinical Scenarios

Pregnancy

• Use with caution in the first trimester, though lactulose is the preferred osmotic laxative when needed during pregnancy 2, 5
• Excessive use can cause maternal bloating 2

Palliative Care

• For severe constipation with life expectancy of months to weeks, the National Comprehensive Cancer Network recommends 30-60 mL BID-QID, increasing every few days based on response 5
• In dying patients (weeks to days life expectancy), combine lactulose with senna ± docusate, targeting 1 non-forced bowel movement every 1-2 days 5

Diabetic Patients

• Monitor glucose carefully due to the carbohydrate content of lactulose, which may require adjustment in diabetes management 5

Pediatric Dosing

• Infants: 2.5-10 mL daily in divided doses 3
• Older children and adolescents: 40-90 mL total daily dose 3
• For disimpaction in children: 4-6 mL/kg/day (max 120 mL/day) until resolution or up to 6 days 7

Key Clinical Pitfalls to Avoid

• Do not use lactulose as first-line therapy when over-the-counter options (polyethylene glycol, fiber) haven't been tried, as it is a second-line agent 5
• Avoid alkaline enemas (soap suds) if using lactulose rectally, as they interfere with its mechanism of action 3
• Do not continue at high doses if diarrhea develops—reduce dose immediately or discontinue 3
• Be aware that long-term safety data is lacking, as the evidence base consists primarily of short-term studies 2

Cost Consideration

• Lactulose is cost-effective at less than $50 per month, making it accessible compared to prescription secretagogues (linaclotide $523/month, plecanatide $526/month, prucalopride $563/month) 1