Lactulose is used in the treatment of chronic constipation^[2]. The metabolites of lactulose draw water into the bowel, causing a cathartic effect through osmotic action. It is safe for people of all ages, except for those in a very small percentage of the population that are galactose intolerant. Dosage may have to be adjusted over time to produce the desired effect because the laxative effect can decline with daily treatment.

Lactulose works by increasing the water content and volume of the stools in the bowel, making them softer and easier to pass. Lactulose is made up of sugar molecules and is partly broken down by the bacteria that live in the lower part of the gut. This leads to the contents of the gut becoming more acidic because there are carboxylic acids among its metabolites, which are not absorbed from the lower bowel. That in turn causes water to be retained in the lower bowel. This increases the amount of water in the stools, softening them and making them easier to pass. The added volume of gas and flatulence caused by fermentation of the lactulose adds to the volume of the fecal material, and makes it easier to expel. The slightly acidic condition of the colon caused by the presence of acids causes active peristalsis. The combined effect of all of these processes relieves constipation in a very effective manner.

Lactulose is useful in treating people with chronic constipation due to forgetting or ignoring the need for defecation. The treatment is to start the dosage at a low amount and then gradually increase it over a period of weeks until the urgency is such that a daily bowel movement cannot possibly be ignored. After an efficacious dosage is determined, it is maintained indefinitely or increased over time, as required. Lactulose is useful in treating people who abuse laxatives provided that they use lactulose as their only laxative drug, because it has very low toxicity yet produces powerful results.