Lactulose (brand name Portalak) is a synthetic, non-digestible sugar used in the treatment of chronic constipation and hepatic encephalopathy, a complication of liver disease. It is a disaccharide (double-sugar) formed from one molecule each of the simple sugars (monosaccharides) fructose and galactose. Lactulose is not normally present in raw milk, but is a product due to the heat-processes: the more the heat, the more the presence of this substance (from 3.5 mg/l in low temperature pasteurized milk to 744 mg/l in in-container sterilized milk). It is produced commercially by isomerization of lactose.

It is broadly classified as an osmotic laxative.