About lactulose

Lactulose is a laxative taken to treat constipation (difficulty pooing). It is also taken to help a severe liver disease called hepatic encephalopathy.

Lactulose comes as a sweet syrup that you swallow.

It's available on prescription and to buy from pharmacies.

Do not give lactulose to children under 14 years unless recommended by a doctor.

Key facts

  • Lactulose relieves constipation by drawing water into the bowel to make poo softer.
  • The most common side effects are diarrhoea, bloating and wind. These are usually mild and shortlived.
  • Lactulose takes at least 48 hours to work.
  • If you find the taste of lactulose too sweet, you can dilute it with fruit juice or water.
  • Lactulose is also called by the brand names Duphalac and Lactugal.

Who can and can't take lactulose

Lactulose can be taken by adults including pregnant and breastfeeding women. It can also be taken by children aged 14 years and over.

Babies and children can take lactulose if their doctor recommends it. Do not give lactulose to a child under the age of 14 years unless your doctor has said so.

Lactulose isn't suitable for some people. To make sure lactulose is safe for you, tell your doctor or pharmacist if you:

  • have ever had an allergic reaction to lactulose or any other medicine in the past
  • can't digest a sugar called lactose (lactose intolerance)
  • have a rare health problem where the body cannot process a sugar called galactose (galactosaemia)
  • have diabetes (as lactulose may affect your blood sugar levels)

How and when to take it

Take lactulose once or twice a day. You can take it with or without food.

How much to take

Your dose of lactulose can go up or down, depending on how well the medicine is working.


Only give lactulose to children under 14 years if their doctor recommends it.

For constipation in:

  • adults – the starting dose is usually 15ml taken twice a day
  • children aged 5 to 17 years - the usual dose is 5ml to 20ml twice a day
  • children aged 1 to 4 years – the usual dose is 2.5ml to 10ml twice a day
  • babies aged 1 to 11 months – the usual dose is 2.5ml twice a day

For adults with hepatic encephalopathy, the usual dose is between 30ml and 50ml taken 3 times a day.

How to take it

The medicine comes with a plastic cup or spoon to measure your dose. Do not use a kitchen spoon as it will not give the right amount. If you don't have a cup or spoon, ask your pharmacist for one.

Some people don't like the sweet taste of lactulose. To improve the taste, you can mix your dose with half a glass or water or fruit juice.

Aim to drink 6 to 8 glasses of fluid during the day while you are taking lactulose or your constipation may get worse.

What if I forget to take it?

If you forget a dose of lactulose, don't worry, just take the next dose at the usual time.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If your doctor has told you to take lactulose every day and you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

Taking an extra dose of lactulose by accident is unlikely to harm you. You may get diarrhoea and stomach pain but this should ease off within a day or two.

If you're worried, talk to your doctor or pharmacist for advice.

Side effects

Like all medicines, lactulose may cause side effects in some people, but many people have no side effects or only minor ones.

Common side effects

A very common side effect, particularly at high doses, is diarrhoea. This happens in more than 1 in 10 people.

Other common side effects, which happen in more than 1 in 100 people, are:

  • bloating
  • wind (farting and burping)
  • feeling sick
  • being sick (vomiting)
  • stomach pain

These side effects are mild and usually go away after a couple of days. Talk to your doctor or pharmacist if the side effects bother you or don't go away.

Serious side effects

Call a doctor straight away if these unlikely but serious side effects happen to you:

  • severe diarrhoea or vomiting
  • muscle cramps or weakness
  • irregular heartbeat

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to lactulose.

Call your local emergency service if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

You could be having a serious allergic reaction and may need immediate treatment in hospital.

These are not all the side effects of lactulose. For a full list see the leaflet inside your medicines packet.

How to cope with side effects

What to do about:

  • diarrhoea - drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark strong-smelling pee. Reducing your dose of lactulose may also help diarrhoea. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
  • bloating - take lactulose between meals instead of before or after them
  • wind - steer clear of foods that cause wind like lentils, peas, beans and onions. It might also help to eat smaller and more frequent meals, eat and drink slowly, and exercise regularly. Some pharmacy remedies help wind, such as charcoal tablets or simethicone.
  • feeling sick - try taking lactulose with meals, or mixing your dose with some water or fruit juice
  • being sick (vomiting) - drink lots of fluids, such as water or squash, to avoid dehydration. Take small, frequent sips. Do not take any other medicines to treat vomiting without speaking to a pharmacist or doctor.
  • stomach pain - try to rest and relax. It can help to eat and drink slowly and have smaller and more frequent meals. Putting a heat pad or covered hot water bottle on your stomach may also help. If you are in a lot of pain, speak to your pharmacist or doctor.

Pregnancy and breastfeeding

Lactulose is generally safe to take during pregnancy and while breastfeeding.

Constipation is common at the end of pregnancy and just after having a baby.

If you're pregnant or breastfeeding, it's always better to try to safely treat constipation without taking a medicine.

Your doctor or midwife will first advise you to eat more fibre and drink plenty of fluids. You'll also be encouraged to do gentle exercise.

If dietary and lifestyle changes don't work, you may be recommended a laxative such as lactulose.

For more information about how laxatives can affect you and your baby during pregnancy, read this leaflet on the Best Use of Medicines in Pregnancy (BUMPS) website.


Tell your pharmacist or doctor if you're trying to get pregnant, are already pregnant or if you're breastfeeding.

Cautions with other medicines

There are no known problems mixing lactulose with other medicines or herbal remedies.


For safety, tell your doctor or pharmacist if you're taking any other medicines, including herbal remedies, vitamins or supplements.

Common questions