Lactulose is a prescription laxative used primarily for two purposes: treating constipation and managing a serious liver-related brain condition called hepatic encephalopathy. It works differently from stimulant laxatives because it draws water into the colon rather than forcing muscle contractions, making it a gentler option that doctors often recommend for longer-term use.
How Lactulose Works
Lactulose is classified as an osmotic laxative. Your body cannot digest or absorb it, so it passes through your stomach and small intestine completely intact until it reaches your colon. Once there, bacteria break it down into organic acids, mainly lactic acid and acetic acid. These byproducts pull water from surrounding tissues into the bowel, softening stool and making it easier to pass.
This water-drawing mechanism is what separates lactulose from other types of laxatives. Stimulant laxatives trigger the muscles in your intestinal wall to contract, pushing stool along faster. Lactulose takes a slower, more passive approach: it changes the environment inside your colon so that stool naturally becomes softer and bulkier. That’s why it can take 24 to 48 hours to produce results, whereas stimulant laxatives often work within hours.
Treating Constipation
The most common reason people are prescribed lactulose is chronic constipation. The typical adult starting dose is 15 ml taken twice a day, usually adjusted based on how you respond. It comes as a sweet-tasting liquid syrup that you can mix with water or juice if the flavor is too strong.
Unlike some laxatives that are only meant for occasional use, lactulose can be taken regularly over weeks or months. It doesn’t create the kind of dependency associated with stimulant laxatives, where your bowel gradually “forgets” how to move on its own. That said, the goal is always to use it alongside diet and lifestyle changes (more fiber, more water, more movement) rather than as a permanent fix.
Children can also be prescribed lactulose for constipation, with doses adjusted for age and weight. It’s one of the more commonly recommended options in pediatric care because of its relatively gentle mechanism.
Managing Hepatic Encephalopathy
Lactulose’s second major use is less well known but arguably more critical. In people with severe liver disease, the liver can no longer filter toxins from the blood effectively. One of those toxins, ammonia, builds up and crosses into the brain, causing confusion, personality changes, disorientation, and in severe cases, coma. This condition is called hepatic encephalopathy.
Lactulose tackles this problem through several mechanisms working together. When bacteria in the colon break lactulose into organic acids, the resulting drop in pH (the colon becomes more acidic) converts ammonia into a form called ammonium. Ammonium cannot pass back through the intestinal wall into the bloodstream, so it gets trapped in the colon and excreted. At the same time, the more acidic environment pushes out ammonia-producing bacteria and encourages the growth of beneficial bacteria (like Lactobacillus) that don’t produce ammonia. On top of all this, lactulose speeds up bowel movements, giving ammonia less time to be absorbed and increasing the amount of nitrogen expelled in stool.
For hepatic encephalopathy, the dosage is higher than for constipation, and the goal is very specific: producing two to three soft stools per day. Doses are adjusted every day or two until that target is reached. This stool frequency is the clearest sign that enough ammonia is being cleared from the body.
Common Side Effects
Because lactulose works by fermenting in the colon, gas is the most predictable side effect. Bloating, flatulence, a feeling of fullness, and stomach cramps are all reported. These symptoms tend to be worst during the first few days of use and often improve as your gut bacteria adjust to the medication.
Diarrhea can occur if the dose is too high. This is usually a sign to reduce the amount rather than stop entirely. If diarrhea becomes persistent, it can lead to dehydration and electrolyte imbalances, particularly low potassium. People on long-term or high-dose lactulose (especially those being treated for hepatic encephalopathy) should have their electrolyte levels monitored periodically.
How It Compares to Other Osmotic Laxatives
The most common alternative to lactulose is polyethylene glycol (sold under brand names like MiraLAX and ClearLAX). A meta-analysis comparing the two found that they are equally effective at increasing bowel frequency, and both have a low overall incidence of adverse events. Where they differ is in tolerability: polyethylene glycol caused significantly less bloating, flatulence, and abdominal pain than lactulose.
This makes practical sense given how each drug works. Polyethylene glycol draws water into the colon without being fermented by bacteria, so it doesn’t produce the gas that lactulose does. If bloating is a major issue for you, polyethylene glycol may be the more comfortable option. However, lactulose has the advantage of its dual role in treating hepatic encephalopathy, something polyethylene glycol is not approved for. Lactulose also doesn’t require mixing with a specific volume of water the way polyethylene glycol powder does.
Who Should Not Take Lactulose
People with galactosemia, a rare genetic condition where the body cannot process the sugar galactose, should not take lactulose. The medication contains small amounts of galactose and lactose as byproducts of its manufacturing process.
If you have diabetes, it’s worth knowing that lactulose is a synthetic sugar. While only a small amount is absorbed into the bloodstream, high doses (particularly those used for hepatic encephalopathy) could have a modest effect on blood glucose levels. This doesn’t necessarily rule lactulose out, but it’s something to factor in when managing your blood sugar.
People who are already experiencing nausea, vomiting, or any signs of a bowel obstruction should not take lactulose or any other laxative until the cause has been identified. Adding more fluid and bulk to a blocked intestine can make the situation worse.
What to Expect When Taking It
If you’re starting lactulose for constipation, don’t expect immediate results. It typically takes one to two days before you notice a change, and it can take up to three days to reach full effect. This is normal and doesn’t mean it isn’t working. The bacteria in your colon need time to break the lactulose down and generate the acids that draw water into the bowel.
The liquid has a distinctly sweet taste that some people find unpleasant. Mixing it with half a glass of water, fruit juice, or milk can make it easier to get down. Store it at room temperature and avoid freezing it, as this can change its consistency. Lactulose does not need to be taken with food, though some people find it causes less bloating when taken alongside a meal.