Pharmacology: In the colon, lactulose is broken down by colonic bacteria into low molecular organic acids. These acids lead to a lowering of pH in the colonic lumen and via an osmotic effect to an increase of the volume of the colonic contents.
These effects stimulate the peristalsis of the colon and normalize the consistency of the stools. The constipation is cleared and the physiological rhythm of the colon is reinstated.
In portal systemic encephalopathy (PSE) c.q. (pre) coma hepaticum, the effect has been attributed to suppression of proteolytic bacteria by an increase of acidophilic bacteria (eg, lactobacillus), trapping of ammonia in the ionic form by the acidification of the colonic contents, catharsis due to the low pH in the colon as well as an osmotic effect and alteration of the bacterial nitrogen metabolism by stimulating the bacteria to utilize ammonia for bacterial protein synthesis. Within this context, however, it should be realized that hyperammonia alone cannot explain the neuropsychiatric manifestations of PSE.
The ammonia however might serve as a model compound for other nitrogenous substances.
Pharmacokinetics: Lactulose is scantily absorbed after oral administration. Not being absorbed as such, it reaches the colon unchanged. There, it is metabolized by the colonic bacterial flora. Metabolism is complete at doses up to 25-50 g (40-75 mL): At higher dosages, a part may be excreted unchanged.