Lactulose is a semi-synthetic disaccharide composed of fructose and galactose. It is not absorbed in the intestine. It is broken down into osmotically more active products by the intestinal flora. This causes osmotic diarrhoea.
Hence it is used in the treatment of constipation. 10 gm twice daily is the dosage. It softens stools in one to three days.
Use in hepatic precoma
The products of lactulose degradation decreases the pH of the intestine. This converts the ammonia produced by the intestinal flora into NH4+ ions, thus decreasing the absorption. Hence it is used in the treatment of hepatic coma (porto-systemic encephalopathy). For this a higher dose of 20 gm three times daily or more may be needed.