Lactulose is a disaccharide: a sugar molecule composed of two smaller sugar molecules bonded together, in this case fructose and galactose. (The suffix “ose” denotes a sugar molecule.) Mammals and birds are not able to digest lactulose so it passes unabsorbed from mouth all the way down to the large intestine. There, the resident large numbers of bacteria are able to digest the lactulose and consume it as their own food. In doing so these bacteria produce lactic, acetic, and formic acid and carbon dioxide gas. The acids biochemically draw fluid into the bowel to soften the stool, hence the common use of lactulose as a laxative. The acidification of the colon contents attracts ammonia from the bloodstream and traps it there to be excreted with the stool. This is very helpful for patients in liver failure as their diseased livers are not able to detoxify ammonia; excess ammonia in the blood stream leads to a form of dementia called “hepatic encephalopathy.”