Lactulose is a semi-synthetic disaccharide used in the treatment of constipation and hepatic encephalopathy. It consists of the monosaccharides fructose and galactose. In the colon, lactulose is broken down primarily to lactic acid, and also to small amounts of formic and acetic acids, by the action of b-galactosidase from colonic bacteria, which results in an increase in osmotic pressure and slight acidification of the colonic contents. This in turn causes an increase in stool water content and softens the stool. In treating hepatic diseases (hepatic encephalopathy), lactulose draws out ammonia from the body in the same way that it draws out water into the colon.