Lactulose: Osmotic Laxative and Treatment for Hepatic Encephalopathy
Lactulose is a synthetic disaccharide consisting of galactose and fructose that is not digested or absorbed in the human small intestine. It reaches the colon intact, where it is fermented by colonic bacteria, generating organic acids and gases. This dual property makes it an effective osmotic laxative and a cornerstone of treatment for hepatic encephalopathy.
Lactulose has been available since the 1950s and despite the availability of newer laxatives, remains widely used due to its established safety profile, effectiveness in hepatic encephalopathy, and suitability in special populations including pregnancy and the elderly. Its onset of action is 24 to 48 hours, making it suitable for chronic constipation management rather than acute relief.
Mechanism of Action
In the colon, lactulose is metabolised by bacteria (primarily Lactobacillus and Bifidobacterium species) into short-chain fatty acids (acetate, propionate, butyrate), lactic acid, and CO2. The accumulating organic acids lower colonic pH and increase osmotic pressure within the colon, drawing water into the intestinal lumen and softening the stool. The increased stool volume stimulates peristalsis. For hepatic encephalopathy, the mechanism involves additional effects: the acidified colonic environment converts ammonia (NH3) to ammonium (NH4+), which is charged and cannot be absorbed back into the blood; the cathartic effect reduces the colonic transit time, limiting ammonia production; and the altered pH environment may reduce the growth of urease-producing bacteria that generate ammonia from dietary protein and urea.
Indications
Lactulose is approved for the treatment and prevention of chronic constipation and for the treatment of portal-systemic (hepatic) encephalopathy in patients with liver cirrhosis. In constipation management, it is particularly suitable for elderly patients, those on long-term opioid therapy, and pregnant women (it is not systemically absorbed). For hepatic encephalopathy, lactulose reduces blood ammonia levels and improves cognitive function and consciousness. It is used both in acute episodes and for long-term prevention of recurrence in patients with cirrhosis.
Dosage and Administration
For constipation: 10 to 20 mL (6.7 to 13.3 g) once or twice daily; dose adjusted to produce two to three soft stools per day. For hepatic encephalopathy: 30 to 50 mL three to four times daily, titrated to produce two to three soft stools per day; higher doses are used in acute hepatic encephalopathy aiming for four to six stools daily. Lactulose solution should be diluted in water or juice to improve palatability. Lactulose is also available as powder sachets. Adequate fluid intake must be maintained during treatment.
Side Effects
Flatulence, bloating, and abdominal cramps are the most frequent adverse effects, particularly during the first weeks of treatment as the gut microbiome adapts. Diarrhoea occurs with excessive doses and is the dose-limiting side effect. Nausea occurs in some patients, particularly with the sweet taste of the solution. Electrolyte disturbances (hypokalaemia, hypernatraemia) can develop with prolonged diarrhoea or high doses. In patients with diabetes or lactose intolerance, the small residual amounts of galactose and lactose in commercial preparations may be relevant; lactulose-specific products with lower galactose content are available. Aspiration of lactulose solution should be avoided in patients with swallowing difficulties.
Interactions
Neomycin and other gut-active antibiotics can interfere with the colonic bacterial fermentation that generates the acidic products necessary for lactulose's effect in hepatic encephalopathy; however, the combination with rifaximin is established and beneficial. Antacids containing non-absorbable antacids may reduce the colonic acidification effect. Other laxatives combined with lactulose increase diarrhoea risk. In patients on insulin or oral hypoglycaemics, the small carbohydrate content of lactulose is generally negligible at therapeutic doses but should be considered with very high doses.
Special Notes
Lactulose contains small amounts of lactose and galactose; patients with galactosaemia or lactose intolerance who are very sensitive should use lactulose with caution or choose alternative preparations. Lactulose is generally safe in pregnancy and breastfeeding as it is not systemically absorbed. In hepatic encephalopathy, lactulose therapy is a lifelong maintenance treatment in patients with cirrhosis who have had an episode of overt encephalopathy; discontinuation leads to high recurrence rates. Rifaximin combined with lactulose is more effective than lactulose alone for prevention of recurrent hepatic encephalopathy.
Related Topics
Frequently Asked Questions
How long does lactulose take to work?
Lactulose has an onset of action of 24 to 48 hours, as the drug must reach the colon and undergo bacterial fermentation before the osmotic and acidification effects develop. This delayed onset means lactulose is not suitable for immediate relief of acute constipation. It is best suited for the prevention and management of chronic constipation.
Why is lactulose used for hepatic encephalopathy?
In liver cirrhosis, the liver cannot detoxify ammonia produced by gut bacteria from dietary protein and urea. Ammonia crosses the blood-brain barrier and causes the neurological symptoms of hepatic encephalopathy. Lactulose lowers blood ammonia by acidifying the colon (trapping ammonia as non-absorbable ammonium), accelerating transit (less time for ammonia absorption), and altering the gut microbiome to reduce ammonia production.
Can lactulose be used during pregnancy?
Yes, lactulose is considered safe during pregnancy as it is not absorbed from the gut and has no systemic effects on the mother or foetus. It is one of the preferred treatments for constipation in pregnancy, alongside adequate dietary fibre and fluid intake. It can also be used during breastfeeding for the same reason.
Sources
- EMA: Lactulose Summary of Product Characteristics 2023
- EASL Clinical Practice Guidelines on Hepatic Encephalopathy 2022
- World Gastroenterology Organisation Global Guidelines on Constipation 2023