An osmotic laxative

Lactulose

An osmotic laxative that softens stools by drawing water into the bowel; it takes a day or two to work and is also used in liver disease.

What is Lactulose?

Lactulose is an osmotic laxative used to treat constipation by drawing water into the bowel, which softens the stools and makes them easier to pass. It can take a day or two to start working, so it is not for immediate relief, and drinking plenty of fluids helps it work. It is also used in liver disease to help clear waste products that can build up.

Education and reference only. This is a plain-language guide to Lactulose — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.

Written by Meds Global Health Clinical Team · Clinically reviewed by Dr Muhammad Suffyan, MB BS, Founder & Clinical Director · Last reviewed · How we review
Class: Laxatives → Brands: Duphalac
Lactulose (Laxatives) — Meds Global Health reference card with 2D molecular structure
Lactulose — Laxatives. The image shows the active ingredient's 2D molecular structure.

What it is

Lactulose is an osmotic laxative used in the UK to treat constipation, and also in liver disease (hepatic encephalopathy) to help the body get rid of waste products. As a laxative it works by drawing water into the bowel to soften the stools and make them easier and more comfortable to pass. It is a gentle, gradual treatment rather than a fast-acting one, so it usually takes a day or two to take effect, and it is taken regularly while it is needed.

How it works

Lactulose is a type of sugar that the body does not absorb, so it passes through to the large bowel. There it draws water into the gut (an osmotic effect), which softens the stools and increases their bulk, helping them move along. Bacteria in the bowel also break it down, which is part of how it helps in liver disease by encouraging the removal of ammonia, a waste product that can build up and affect the brain.

Practical use

How to take Lactulose

General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.

  • Take it regularly as advised; remember it usually takes a day or two to work, so do not expect immediate relief.
  • Drink plenty of fluids while taking it, which helps it work and keeps the stools soft.
  • Expect some wind, bloating or cramps at first; these often ease as your body gets used to it.
  • You can mix the solution with a drink of water or fruit juice if you find the taste too sweet.
  • Keep taking it for as long as advised, and review with your prescriber whether you still need it.

Weighing it up

Advantages & disadvantages of Lactulose

Advantages

  • It is a gentle, gradual laxative that softens the stools and is comfortable to use.
  • It can be used for longer periods and is suitable in pregnancy.
  • It has an extra role in liver disease, helping clear waste products that can build up.

Disadvantages

  • It is slow to work, taking a day or two, so it is not suitable when quick relief is needed.
  • It commonly causes wind, bloating and cramps, particularly at the start.
  • The sweet taste does not suit everyone, and it needs to be taken with plenty of fluids.

Practical use

Good to know

Lactulose usually takes a day or two to start working, so it is not the right choice when you need quick relief — it builds up its effect gradually. Drinking plenty of fluids helps it work and is important to avoid the stools becoming hard. Because bacteria in the bowel ferment it, it commonly causes wind (flatulence), bloating and tummy cramps, especially at first; these often settle with time. It is generally safe to use for longer periods and is suitable in pregnancy, and a different, higher-strength regimen is used by specialists in liver disease.

Who should not take it / use with caution

  • People with a suspected or known bowel blockage (obstruction).
  • People with galactosaemia (a rare inability to handle galactose), as the solution contains related sugars.
  • Used with caution in people who cannot tolerate certain sugars, such as lactose intolerance, who may get more wind and bloating.

Monitoring

  • Review of whether constipation has improved and whether the laxative is still needed.
  • Checking that enough fluid is being taken so the stools stay soft.
  • In liver disease, closer specialist monitoring of bowel response and the condition itself.

Side effects

  • Wind (flatulence), bloating and tummy cramps, especially when starting.
  • Nausea, particularly with larger amounts.
  • Loose stools or diarrhoea if too much is taken.
  • Rarely, an imbalance of salts in the body if very large amounts are used over a long time.

Key interactions

  • It has few important interactions with other medicines.
  • Other laxatives taken at the same time can add to its effect and cause cramps or diarrhoea.
  • In liver disease, certain antibiotics that change gut bacteria can affect how well it works; this is managed by a specialist.

Available as: An oral solution.

Answers

Lactulose: frequently asked questions

How long does lactulose take to work?

It usually takes a day or two to start working, as it builds up its effect gradually rather than acting quickly. Because of this it is not the right choice when you need fast relief from constipation.

Why does lactulose cause wind and bloating?

Bacteria in the bowel ferment lactulose, which produces gas. This is why it commonly causes wind, bloating and cramps, particularly at first. These effects often settle as your body gets used to it.

Do I need to drink more fluids with lactulose?

Yes — drinking plenty of fluids helps lactulose work by keeping water in the bowel and the stools soft. Without enough fluid, the stools can become hard and harder to pass.

Can I take lactulose in pregnancy?

Lactulose is generally considered suitable in pregnancy and is often used because it works gently and is not absorbed into the body. As always, check with your midwife, pharmacist or doctor if you are unsure.

Why is lactulose used in liver disease?

In liver disease it helps the bowel remove ammonia, a waste product that can build up and affect the brain. A specific, more intensive regimen is used and managed by specialists, which is different from its use for ordinary constipation.

The wider class

About Laxatives

Lactulose belongs to the laxatives class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.

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Authoritative sources

  • BNF
  • NICE CKS

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