Is a 10‑gram dose of lactose an appropriate therapeutic dose for treating constipation in an adult?

Recommended Dose of Lactulose for Constipation

A 10-gram dose of lactulose is below the FDA-approved therapeutic range and should not be used; the recommended initial dose is 15 grams (10–20 g range) daily, which can be titrated up to 40 grams daily if needed. 1

FDA-Approved Dosing

• Lactulose is FDA-approved for constipation at an initial dose of 10–20 grams (15–30 mL or 1–2 packets) daily 1
• The recommended starting dose is 15 grams daily according to the 2023 AGA-ACG guidelines 1
• The dose may be increased to 40 grams (60 mL or 2–4 packets) daily if the initial dose is insufficient 1
• A 10-gram dose falls at the very bottom of the therapeutic range and is likely subtherapeutic for most adults with chronic constipation

Clinical Context and Positioning

• Lactulose should be reserved for patients who fail or are intolerant to over-the-counter therapies (conditional recommendation, very low certainty evidence) 1
• Polyethylene glycol (PEG) 17 grams daily is strongly preferred as first-line pharmacologic therapy over lactulose (strong recommendation, moderate certainty evidence) 1, 2
• PEG is superior to lactulose in stool frequency per week, stool form, relief of abdominal pain, and need for additional products 3

Mechanism and Efficacy

• Lactulose is a synthetic disaccharide that exerts an osmotic laxative effect by remaining undigested in the small intestine and drawing water into the colon 1
• At therapeutic doses (15–40 g), lactulose is fermented by colonic bacteria to lactic and acetic acids, which lower colonic pH and stimulate propulsion 4
• Lactulose at 10 grams twice daily (20 g total) significantly accelerates ascending colon emptying compared to PEG in healthy subjects 5
• However, evidence for lactulose efficacy is very low quality, based on two small RCTs from over 40 years ago in elderly nursing home populations 1

Dose-Dependent Adverse Effects

• Bloating and flatulence are dose-dependent and represent the most common side effects that limit lactulose use in clinical practice 1
• These symptoms may be particularly limiting at higher doses or in patients with preexisting bloating or abdominal pain 1
• A 10-gram dose may produce fewer GI side effects but is unlikely to provide adequate therapeutic benefit

Practical Algorithm for Lactulose Use

Step 1: Confirm that PEG 17 g daily has been tried and either failed or was not tolerated 1, 2

Step 2: Start lactulose at 15 grams daily (not 10 grams), taken as 15 mL of syrup or one packet 1

Step 3: Titrate the dose upward per symptom response and side effects over 1–2 weeks 1

Step 4: If 15 grams is insufficient and well-tolerated, increase to 30 grams daily, then up to 40 grams daily as needed 1

Step 5: If bloating or flatulence become limiting, consider switching back to PEG or adding a stimulant laxative (bisacodyl 5 mg or senna 8.6–17.2 mg) for short-term or rescue use 1

Special Considerations

• Lactulose is the only osmotic agent studied in pregnancy and may be preferred in pregnant patients with constipation 1
• In mildly constipated, non-insulin-dependent patients with type 2 diabetes, lactulose may not increase blood sugar levels 1
• Generic lactulose is generally low cost (<$50/month), though some branded formulations may be expensive 1

Key Pitfall to Avoid

Do not prescribe a 10-gram dose of lactulose as monotherapy for constipation—this dose is at the extreme lower end of the FDA-approved range and is unlikely to provide meaningful clinical benefit. Start at 15 grams daily and titrate upward based on response, or preferentially use PEG 17 grams daily as first-line therapy. 1