How does lactulose relieve constipation?

Mechanism of Action of Lactulose for Constipation Relief

Lactulose relieves constipation primarily by acting as an osmotic laxative, drawing water into the intestine to hydrate and soften stool, while also promoting peristalsis through colonic fermentation. 1

Primary Mechanism of Action

• Lactulose is a synthetic disaccharide that functions as an osmotic laxative by drawing water into the intestinal lumen, which hydrates and softens the stool 1
• Being non-digestible by human enzymes, lactulose reaches the colon intact where it is fermented by colonic bacteria 2, 3
• This fermentation process produces organic acids and gases that further enhance its laxative effect 2
• Lactulose produces a prolonged tonic contraction in the colon that contributes to its laxative effect 4

Secondary Effects Contributing to Efficacy

• The bacterial fermentation of lactulose in the colon decreases the pH of the colonic environment 3
• The acidification of colonic contents increases peristaltic activity, further promoting bowel movements 3
• Lactulose increases stool frequency, weight, volume, and water content while producing softer stool consistency compared to baseline 5
• The laxative effects of lactulose appear to be dose-related, with higher doses producing stronger effects 5

Dosing Considerations

• For constipation treatment, the recommended adult oral dosage is 10-20 g daily (approximately 2-3 tablespoonfuls), which may be increased to 40 g daily if necessary 6, 3
• The American Gastroenterological Association recommends starting with a lower dose and increasing as tolerated to minimize gastrointestinal side effects 3
• Therapeutic effects may begin within 24-48 hours but sometimes take longer to manifest 6

Clinical Evidence of Effectiveness

• Studies show that lactulose significantly increases stool frequency, weight, volume, and water content compared to placebo in both normal and constipated subjects 5
• Lactulose accelerates colonic transit time compared to control groups 4
• In hospitalized patients, even a small dose (10 g per day) of lactulose can prevent slow colonic transit associated with physical inactivity 7
• Comparative studies indicate that polyethylene glycol (PEG) may be more effective than lactulose for chronic constipation treatment 8

Common Side Effects

• Gastrointestinal side effects occur in approximately 20% of patients taking lactulose 2, 3
• Common side effects include abdominal pain, bloating, and flatulence, which are dose-dependent 2
• These side effects are extensions of the pharmacologic action of lactulose and are generally tolerable 5
• Excessive dosage can lead to diarrhea with potential complications including electrolyte imbalances 2

Clinical Application

• Lactulose is recommended as a first-line agent for opioid-induced constipation by the American Gastroenterological Association with strong recommendation and moderate quality evidence 1
• It is widely available over the counter, generally very safe, and inexpensive 1
• Beyond its laxative effects, lactulose may have additional benefits as an indirect antioxidant by increasing hydrogen production in the gut 9