What is lactulose used for?

October 11, 2025 •

4 min read

Lactulose, a synthetic disaccharide sugar, is poorly absorbed in the human gut, allowing it to reach the colon largely intact. Here, it is used by gut bacteria to produce acids that increase stool water content and reduce plasma ammonia, making it effective for treating chronic constipation and hepatic encephalopathy.

Quick Summary

This article explains the two primary uses of lactulose, addressing its dual function as a laxative and a treatment for liver-related brain conditions. It details how the medication works within the colon to soften stool and reduce ammonia levels.

Key Points

Dual Action Medication: Lactulose is prescribed to treat both chronic constipation and a complication of liver disease called hepatic encephalopathy.
Osmotic Laxative: For constipation, it works by drawing water into the colon, softening stools and stimulating bowel movements.
Ammonia Reduction: In hepatic encephalopathy, lactulose lowers blood ammonia by trapping it in the colon, where it is then excreted.
Variable Onset: It is not a fast-acting treatment; expect 24 to 48 hours for the medication to produce a bowel movement for constipation.
Potential Side Effects: Common side effects include gas, bloating, and stomach cramps, which usually lessen over time.
Precautions Required: Lactulose is contraindicated in patients with galactosemia and should be used cautiously in diabetics.
Prescription Only: Unlike some other osmotic laxatives, lactulose requires a doctor's prescription.

Lactulose is a prescription medication with a unique mechanism of action that makes it particularly effective for two distinct medical conditions: chronic constipation and hepatic encephalopathy. Unlike other sugars, it passes through the small intestine largely unabsorbed, allowing it to exert its therapeutic effects directly in the large intestine. Understanding its dual purpose and proper usage is key for patients and caregivers.

The Dual Purpose of Lactulose

Lactulose's utility in treating both constipation and hepatic encephalopathy stems from the way it is metabolized by colonic bacteria. Its fermentation by the gut flora produces organic acids that are responsible for its therapeutic properties.

Lactulose for Constipation

For chronic constipation, lactulose works as an osmotic laxative. Its non-absorbable nature means it draws water into the colon via osmosis, which serves several purposes:

Softens stool: The increased water content in the colon makes the stool softer and easier to pass.
Increases bulk: The added fluid volume increases the bulk of the stool, which helps stimulate bowel movements.

This mechanism differs from stimulant laxatives, which directly provoke intestinal muscle contractions. Lactulose's effect is gentler and often requires 24 to 48 hours to produce a bowel movement. Due to its osmotic effect, it can be a suitable option for long-term management of constipation under a doctor's supervision.

Lactulose for Hepatic Encephalopathy (HE)

In patients with severe liver disease, the liver may fail to detoxify the blood properly, leading to a buildup of toxins like ammonia. When ammonia levels rise, it can affect brain function, causing a condition called hepatic encephalopathy (HE), with symptoms like confusion, memory loss, and personality changes. Lactulose addresses this issue through several key actions within the colon:

Acidifies the colon: The fermentation of lactulose by bacteria produces lactic acid and other organic acids, which lowers the pH of the colon contents.
Traps ammonia: This acidic environment converts the toxic, absorbable ammonia (NH₃) into non-absorbable ammonium ions (NH₄⁺), effectively trapping it in the colon.
Increases excretion: The cathartic effect of lactulose then helps expel the trapped ammonium ions from the body through more frequent bowel movements.

For acute episodes of HE, lactulose can be administered orally or as a retention enema for a faster effect. It is considered the gold standard for treating and preventing HE, often used in conjunction with other therapies like the antibiotic rifaximin.

Important Considerations for Taking Lactulose

Dosage and Administration

Dosage varies depending on the condition being treated and is adjusted based on a patient's response and medical professional's guidance. The specific amount and frequency are determined by a healthcare provider. The medication is available in different forms, such as syrup or powder.

Tips for administration: The syrup can have a sweet, syrupy taste, which can be improved by mixing it with water, fruit juice, or milk. The powder form should be dissolved in a liquid before consumption.

Side Effects and Precautions

Common side effects, especially during the first few days of therapy, include flatulence, bloating, burping, and abdominal cramps. These typically subside with continued use. Diarrhea may occur and can lead to fluid and electrolyte imbalances, such as hypokalemia and hypernatremia, if left unaddressed.

Precautions

Diabetes: Lactulose contains some free lactose and galactose and should be used with caution in diabetic patients due to the potential for slight blood sugar increases.
Galactosemia: It is contraindicated in patients who require a low-galactose diet due to the presence of galactose.
Colonoscopy: A theoretical risk of an explosive reaction due to hydrogen gas accumulation exists during electrocautery procedures, so bowel cleansing with a non-fermentable solution is advised beforehand.

Lactulose vs. Other Common Laxatives

Different types of laxatives work in different ways and are suitable for different situations. Below is a comparison of lactulose with two other common types: polyethylene glycol (PEG) and senna.


Feature	Lactulose (Osmotic Laxative)	Polyethylene Glycol (PEG, Osmotic Laxative)	Senna (Stimulant Laxative)
Mechanism	Osmotically draws water into the colon, is fermented by bacteria, and acidifies the bowel contents.	Draws water into the colon via osmosis, increasing stool water content.	Stimulates the muscles of the intestinal wall to promote a bowel movement.
Prescription Status	Prescription required.	Often available over-the-counter (OTC).	Typically available OTC.
Time to Work	24 to 48 hours for effect.	1 to 3 days for effect.	6 to 12 hours for effect.
Common Side Effects	Bloating, gas, cramps, and diarrhea, especially when first starting.	Less likely to cause bloating and gas than lactulose.	Can cause stomach cramps and is not for long-term use.
Additional Use	Manages hepatic encephalopathy by reducing blood ammonia levels.	No additional therapeutic use beyond laxative effect.	No additional therapeutic use beyond laxative effect.
Best For	Chronic constipation and hepatic encephalopathy.	General constipation, often preferred for its milder side effect profile.	Occasional, short-term relief of constipation.

Conclusion

Lactulose is a versatile and effective medication primarily used to manage chronic constipation and hepatic encephalopathy. Its unique mechanism of action, leveraging fermentation by colonic bacteria to draw water into the bowel and trap ammonia, distinguishes it from other laxatives. While often used for long-term care under medical supervision, patients should be mindful of potential side effects, such as gas and bloating, particularly at the start of treatment. Always consult with a healthcare provider to determine if lactulose is the right treatment and to establish the correct dosage based on your specific condition.

For more detailed pharmacological information on lactulose, consult reliable medical resources such as the US National Library of Medicine National Institutes of Health.

Frequently Asked Questions

Lactulose is an osmotic laxative. It is a synthetic sugar that is not absorbed in the small intestine. Once it reaches the colon, bacteria ferment it, producing organic acids. These acids increase osmotic pressure, drawing water into the colon, which softens the stool and promotes a bowel movement.

Both lactulose and Miralax (polyethylene glycol) are osmotic laxatives, but lactulose requires a prescription and is also used for hepatic encephalopathy. Lactulose can cause more side effects like gas and bloating due to bacterial fermentation, while Miralax is less likely to cause these and is available over-the-counter.

In patients with severe liver disease, toxins like ammonia can build up and lead to hepatic encephalopathy. Lactulose's bacterial fermentation in the colon creates an acidic environment that converts ammonia into a non-absorbable form (ammonium ion), trapping it in the gut to be expelled through stool.

Lactulose is not for immediate relief. It typically takes 24 to 48 hours to produce a normal bowel movement.

Common side effects include gas, bloating, stomach cramping, and diarrhea. These often occur during the first few days of treatment and usually lessen with time. Excessive dosage can lead to severe diarrhea and fluid/electrolyte imbalances.

Yes, lactulose is often used for the long-term management of chronic constipation. Unlike stimulant laxatives, it is not habit-forming. However, prolonged use should be under medical supervision to monitor for electrolyte imbalances.

Patients with galactosemia should not take lactulose because it contains galactose. It should also be used with caution in diabetics due to the presence of other sugars. For pregnant or breastfeeding individuals, it is best to consult a doctor first.