saudigastro.com/patients/lactulose
This article is educational and does not replace your doctor's advice. Seek urgent care for any red-flag symptoms.
Drug class
Osmotic laxative (non-absorbable sugar)
Examples
Lactulose syrup/solution
How it works
Lactulose is not absorbed in the small intestine. It draws water into the bowel, which softens stools and makes them easier to pass. In the colon, bacteria break it down into acids that make the gut contents more acidic. In people with advanced liver disease this helps reduce the build-up of ammonia and other toxins that the liver can no longer clear, which can cloud thinking (hepatic encephalopathy).
What it treats
- Constipation, including long-term constipation
- Treating and preventing hepatic encephalopathy (confusion or drowsiness from liver disease)
- Constipation in children or older adults, when advised by a doctor
How to take it
It is taken by mouth as a liquid, measured with the cup or spoon supplied, with or without food, and can be mixed with water or juice. The amount depends on why you take it and is set by your doctor. For constipation it can take a day or two to work. For liver disease, doctors usually aim for about two to three soft stools a day and adjust the dose to reach that; do not change the dose yourself, and tell your doctor if you have too few or too many stools. Drink fluids normally unless told to limit them.
Common side effects
- Gas, bloating and stomach rumbling (often settle after a few days)
- Stomach cramps
- Loose stools or diarrhoea if the dose is too high
- Nausea or a sweet taste
Serious side effects — call your doctor
Severe or watery diarrhoea lasting more than a day or two, dizziness, very dark urine or passing little urine
Signs of dehydration or salt imbalance, which can worsen confusion in liver disease. Contact your doctor the same day.
Severe stomach pain, repeated vomiting, a swollen hard belly or being unable to pass wind or stools
Could be a bowel blockage. This is an emergency: go to the emergency department.
Blood in the stool or black stools
Needs urgent assessment; constipation medicine will not explain this. Seek medical care promptly, and urgently if large amounts or you feel faint.
If you take it for liver disease: new confusion, severe sleepiness, or day-night reversal
May mean hepatic encephalopathy is worsening. Family should seek urgent medical care; do not wait.
Interactions
Lactulose has few direct drug interactions. Other laxatives taken together can cause too much diarrhoea. In people with diabetes it contains small amounts of sugars (such as galactose and lactose), so tell your doctor if you have diabetes or are lactose intolerant. In liver disease, diarrhoea from any cause, or water pills, can cause dehydration, so tell your doctor about all your medicines.
Monitoring
For constipation, your doctor mainly asks how your stools are. For liver disease, your team will watch your thinking, bowel habit, fluid balance and blood salts, and adjust the dose accordingly.
When and how to stop
For constipation, it can usually be stopped when stools are regular, as your doctor advises. For liver disease, do not stop it unless your doctor tells you to, because confusion can return. Ask for advice if you have persistent diarrhoea.
Pregnancy & breastfeeding
Lactulose is not absorbed into the body and is generally considered suitable in pregnancy and breastfeeding when needed. Check with your doctor first.
Questions to ask your doctor
Save this list or print it before your appointment — doctors appreciate prepared patients.
- How much should I take, and when should I change the amount?
- How many bowel movements a day should I aim for?
- Can I take it with my other medicines or if I have diabetes?
- What signs mean I should call you or go to the hospital?
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Status: Approved for publication by SGA; individual clinician review is ongoing · Published 24/04/2026
For general education only; it does not replace your doctor’s advice. Individual clinician review of this guide is ongoing. SGA clinician? Review this guide