About Lactulose A synthetic Disaccharide, Osmotic Laxative, Ammonium Detoxicant. Mechanism of Action of Lactulose Laxative: Lactulose is a synthetic disaccharide of galactose and fructose that resist intestinal disaccharidase activity. The sugars are joined by a beta glycosidic linkage and are resistant to hydrolysis by human digestive enzymes. Lactulose is fermented by a limited number of colonic bacteria. This can lead to changes in the colonic ecosystem in favor of some bacteria, such as lactobacilli and bifidobacteria, which may confer some health benefits. The products of fermentation include lactic acid and small-chain fatty acids. Degradation to short chain fatty acids also increases the osmotic pressure in the intestinal lumen. It also cause an increase in fecal volume due to intraluminal water retention and stimulate colonic propulsive motility result in increased fecal bulk and stimulation of peristalsis.