Lactulose is used in preventing and treating clinical portal-systemic encephalopathy; first used in clinical practice in 1966.[1] Its chief mechanism of action is by decreasing the intestinal production and absorption of ammonia. It has also gained popularity as a potential therapeutic agent for the management of subacute clinical encephalopathy.[2] It is also a laxative for the treatment of chronic constipation and has undergone study as early as the 1960s.[3]  Its osmotic effect and its effect on intestinal motility receive credit for its therapeutic efficacy.[4]

Lactulose can be useful for chronic constipation as a third-line agent, once lifestyle modifications and increasing fiber intake have failed.[4] Because of its ability to significantly reduce intestinal transit time, it also possesses the ability to reduce hyper-saturation of deoxycholic acid, thereby inhibiting cholesterol stone formation.[5] Recent studies have looked at lactulose for the development of novel anticancer therapeutic agents, due to its ability to bind to galectins (a carbohydrate binding protein that plays a role in tumor progression).[6]

Lactulose, also known as 1,4 beta galactoside-fructose, is a non-absorbable synthetic disaccharide made up of galactose and fructose.[7]  The human small intestinal mucosa does not have the enzymes to split lactulose, and hence lactulose reaches the large bowel unchanged.  Lactulose is metabolized in the colon by colonic bacteria to monosaccharides, and then to volatile fatty acids, hydrogen, and methane. Lactulose reduces intestinal ammonia production and absorption in three ways. First, the colonic metabolism of sugars causes a laxative effect via an increase in intraluminal gas formation and osmolality which leads to a reduction in transit time and intraluminal pH. This laxative effect is also beneficial for constipation.[7] Next, lactulose promotes increased uptake of ammonia by colonic bacteria which utilize the trapped colonic ammonia as a nitrogen source for protein synthesis. The reduction of intestinal pH facilitates this process, which favors the conversion of ammonia (NH3) produced by the gut bacteria, to ammonium (NH4+),[8][9][10] an ionized form of the molecule, unable to cross biological membranes. Finally, lactulose also causes a reduction in intestinal production of ammonia. The acidic pH destroys urease-producing bacteria involved in the production of ammonia.  The unabsorbed disaccharide also inhibits intestinal glutaminase activity, which blocks the intestinal uptake of glutamine, and its metabolism to ammonia.

Although a variety of mechanisms of action of lactulose that limit the production and absorption of ammonia in the gut, as explained above, have been reported, it is probable that other laxatives could have the same effects with better tolerability. Lactulose originally received FDA approval in the USA in 1977, but there are concerns regarding the adequacy of data to support its efficacy. As lactulose has been believed to be the therapy for hepatic encephalopathy, it cannot be withheld from patients in need of the therapy from an ethical standpoint. Thus, it remains difficult to conduct human investigation review board approved placebo-controlled trials in the US to confirm or refute the efficacy of lactulose.

Although the oral route (as a syrup) has been the standard mode of administration for the past several decades, it is also effective as a rectal enema.[1][11] Most of these studies were comparisons between lactulose/lactitol enemas and placebo. The few studies that have compared oral vs. rectal lactulose have demonstrated inconclusive findings in the long-term.[12] Lactitol, a second generation disaccharide, is a crystalline powder but is not available in the United States. It is equally efficacious but better tolerated than lactulose and is usually prescribed at a dose of 10 to 90 g per day to cause two soft bowel movements per day.

At present, the most commonly used regimens of lactulose are as follows.[13]

Oral Route:

For constipation, administration of 15-45 ml (or 10-30 gm) 2-4 times daily, until the formation of soft stools.

In patients with hepatic encephalopathy, lactulose is typically given in syrup form at a dose of 15 to 30 mL two to four times a day to aim for two semisoft stools per day.

For acute hepatic encephalopathy, a common option is to administer a bolus of 45 ml (30 gm) and repeat it hourly until the first bowel movement. Once the episode of encephalopathy has subsided, the dose can be titrated to achieve 2-3 soft bowel movements on a daily basis.

Rectal Route:

This mode is preferred if there exists any risk of aspiration via the oral route. The preferred route is to administer it as 300ml in 700ml of water and have it retained in the colon for an hour, repeated every 2 hours until the episode resolves. The patient position should be in the lateral recumbent position to optimize intestinal distribution.

Because lactulose has insignificant absorption by the gut and undergoes rapid excretion by the kidneys, its effects remain localized to the gut microenvironment. Side-effects would include increased bowel sounds (borborygmi), increased flatus and a sensation of bloating. Since its intended use is to soften the stool quantity and increase the stool amount, its most significant side effect remains as diarrhea. The diarrhea is dose-dependent and decreases in severity with a reduction in the dose of lactulose.[7]

The following are the contraindications of lactulose:

1) Patients with galactosemia: Lactulose, as a result of its chemical composition, contains galactose and is contraindicated in patients who require a galactose-free diet.[14]

2) Diabetics: Although only a small fraction of lactulose undergoes systemic absorption, diabetes patients must use it with caution due to a potential to cause hyperglycemia in diabetic individuals as have been documented in a few studies.[15][16]

3) Elderly population: Studies comparing the clinical efficacy and safety of other osmotic laxatives such as sorbitol against lactulose have concluded that in the elderly, lactulose causes increased nausea as a side effect. Sorbitol is thus a safer and an inexpensive drug to use in this population.[17]

4) Pregnancy: Pregnancy is a stressor of hemodynamic physiology. Theoretically, via its osmotic action, the prolonged use of lactulose may lead to electrolyte imbalances.[16]