lactulose

Lactulose is a synthetic disaccharide composed of galactose and fructose that is not hydrolyzed by human digestive enzymes. When ingested, it reaches the colon unchanged where colonic bacteria metabolize it into short‑chain fatty acids, principally lactic acid and acetic acid. This acidification lowers colonic pH, promotes the conversion of ammonia to ammonium, and increases osmotic pressure, drawing water into the lumen.

Clinically, lactulose is used primarily as an osmotic laxative for the treatment of constipation and as a nitrogen‑reducing agent in hepatic encephalopathy. In constipation, doses of 15–30 mL (10–20 g) are taken once or twice daily, adjusted to achieve soft stools without abdominal cramping. For hepatic encephalopathy, the typical adult regimen begins with 25 mL (≈20 g) three times daily, titrated to produce two to three soft stools per day, thereby decreasing serum ammonia levels.

Pharmacokinetically, lactulose is poorly absorbed, and its metabolites are excreted in the feces. The onset of laxative effect occurs within 12–24 hours. Common adverse effects include flatulence, abdominal bloating, and diarrhoea. Rarely, electrolyte disturbances such as hyponatremia or hypokalemia may develop, particularly with prolonged high‑dose use.

Contraindications encompass known hypersensitivity to lactulose, galactosemia, and intestinal obstruction. Caution is advised in patients with diabetes, as lactulose contains fermentable carbohydrates that may affect glycaemic control. Lactulose is classified as pregnancy category B; it is considered safe for use when the potential benefit justifies the minimal risk.

Commercially, lactulose is available in liquid, syrup, and powder formulations under various brand names worldwide. It is listed on the World Health Organization’s Essential Medicines List for both constipation and hepatic encephalopathy.