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Lactulose: Uses, Mechanism & Dosage

Lactulose is a synthetic disaccharide used as a laxative and in the treatment of hepatic encephalopathy. It acts osmotically in the colon and is not absorbed by the body.

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Things worth knowing about "Lactulose"

Lactulose is a synthetic disaccharide used as a laxative and in the treatment of hepatic encephalopathy. It acts osmotically in the colon and is not absorbed by the body.

What is Lactulose?

Lactulose is a synthetic disaccharide composed of the sugar molecules galactose and fructose. It is not absorbed in the small intestine and therefore reaches the large intestine (colon) intact, where it exerts its therapeutic effects. Lactulose is available both by prescription and over the counter, typically as a syrup or powder.

Mechanism of Action

In the colon, lactulose is broken down by intestinal bacteria into short-chain fatty acids (mainly lactate and acetate) and gases (carbon dioxide, hydrogen). This process has two key effects:

Osmotic effect: The metabolic by-products draw water into the bowel lumen, increase stool volume, and stimulate bowel movements (peristalsis), resulting in softer and easier-to-pass stools.
Acidification of the colon: The drop in pH promotes the conversion of toxic ammonia (NH₃) into non-absorbable ammonium ions (NH₄⁺), reducing the amount of ammonia absorbed into the bloodstream. This is particularly important in the treatment of hepatic encephalopathy.

Indications

Constipation

Lactulose is widely used for the treatment of acute and chronic constipation. It is especially suitable for patients who require a gentle laxative, such as elderly patients, pregnant women, and children.

Hepatic Encephalopathy

In patients with liver disease, the liver is unable to adequately detoxify ammonia produced in the gut. Elevated blood ammonia levels can lead to altered consciousness and neurological symptoms (hepatic encephalopathy). By acidifying the gut contents, lactulose reduces ammonia absorption and is therefore used to prevent and treat this complication.

Dosage and Administration

The dosage of lactulose depends on the indication and individual patient response:

Constipation (adults): Typically 15–30 ml of syrup (equivalent to 10–20 g lactulose) daily, divided into one or two doses. The dose can be adjusted as needed.
Hepatic encephalopathy: Higher doses of up to 90 ml daily (in three to four divided doses) may be required to achieve 2–3 soft stools per day.
Children: Dosage is adjusted according to body weight and age and should be supervised by a healthcare provider.

Lactulose should be taken with adequate fluid intake to support its osmotic effect. The full laxative effect typically occurs within 24–48 hours.

Side Effects

Lactulose is generally well tolerated. Possible side effects include:

Bloating and flatulence (especially at the start of treatment)
Abdominal cramps and nausea
Diarrhea if the dose is too high
Electrolyte imbalances (e.g., low potassium) with prolonged high-dose use, particularly in elderly patients

Contraindications and Precautions

Lactulose should not be used in patients with:

Galactosaemia (a rare inherited disorder of galactose metabolism)
Known hypersensitivity to lactulose or any of the excipients
Bowel obstruction (ileus)

Caution is advised in patients with diabetes, as lactulose may contain small amounts of free galactose and fructose. Long-term use should be accompanied by monitoring of electrolyte levels.

Interactions

Lactulose may influence the absorption of other medications. When used concurrently with antibiotics, the efficacy of lactulose in hepatic encephalopathy may be altered, as antibiotics modify the gut flora responsible for metabolizing lactulose.

References

World Health Organization (WHO) - Model Formulary 2023: Lactulose. Geneva: WHO Press.
Sharma, B.C. et al. - Lactulose and rifaximin for hepatic encephalopathy. New England Journal of Medicine, 2013; 369:1071–1073.
European Medicines Agency (EMA) - Assessment report on lactulose-containing medicinal products. London: EMA, 2021.

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