How fast does lactulose work?

About lactulose

Type of medicine An osmotic laxative
Used for Constipation
Also called Lactugal®
Available as Oral liquid medicine, liquid sachets

Constipation is a common problem. It can mean either going to the toilet less often than usual to empty your bowels, or passing hard or painful stools. Constipation can be caused by a number of things. Not eating enough fibre or not drinking enough fluid can cause constipation. Some conditions (such as pregnancy) can cause constipation, as can a lack of exercise or movement (such as being ill in bed) and some medicines.

Often, increasing the amount of fibre in your diet (such as by eating more fruit, vegetables, cereals, and wholemeal bread) and drinking plenty of water each day can effectively prevent or relieve constipation. You will have been recommended lactulose, a laxative, to help relieve constipation if you cannot increase the fibre in your diet or if this is insufficient. Lactulose works by drawing fluid into your bowel, which makes your stools softer and easier to pass. It is available on prescription, and it can also be bought without a prescription at pharmacies.

Lactulose also changes the acidity of the stools and this helps to discourage the growth of some germs (bacteria) present in the bowel. Because of this, lactulose is also prescribed for people with a liver problem called hepatic encephalopathy. If you have been prescribed it for this reason, ask your doctor if you need further information about the medicine.

Before taking lactulose

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking lactulose it is important that your doctor or pharmacist knows:

  • If you are so constipated that you think you may have a blockage.
  • If you are unable to digest milk sugar (lactose intolerant).
  • If you have a condition called galactosaemia, where your body cannot process galactose.
  • If you are pregnant or breast-feeding. Lactulose is not known to be harmful to a baby, but while you are expecting or feeding a baby, you should only take medicines on the recommendation of a doctor.
  • If it is intended for a child. This is because laxatives should only be given to children on the advice of a doctor or healthcare professional.
  • If you are taking other medicines, including those available to buy without a prescription, as well as herbal and complementary medicines.
  • If you have ever had an allergic reaction to this or to any other medicine.

How to take lactulose

  • Before you start the treatment, read the manufacturer’s printed information leaflet from the pack. It will give you more information about lactulose and will provide you with a full list of the side-effects which you may experience.
  • Take lactulose exactly as your doctor tells you to, or as directed on the pack. The usual dose for constipation in an adult is 15 ml (three 5 ml spoonfuls) twice daily, although the dose you are advised to take may be different from this as it will be adjusted to suit your needs.
  • If a doctor or healthcare professional has recommended lactulose for your child, check the label on the pack carefully to make sure that you give the correct dose for the age of your child.
  • If you forget to take a dose, do not worry, just take the next dose when it is due. Do not take two doses together to make up for a forgotten dose.

Getting the most from your treatment

  • Lactulose can take up to 48 hours to act, so it may take two or three days before you feel the full benefit. However, if after several days you do not feel your symptoms are improving, or if they get worse, you should speak with a doctor for further advice.
  • It is important for you to drink plenty while you are constipated. Adults should aim to drink at least two litres (about 8-10 cups) of fluid per day. Most sorts of drink will do, but as a start, try just drinking a glass of water 3-4 times a day in addition to what you normally drink.
  • Try to eat a balanced diet containing high-fibre foods such as wholemeal and wholegrain breads and cereals, fruit and vegetables, brown rice and wholemeal pasta. If you are not used to a high-fibre diet, it may be best to increase the amount of fibre you eat gradually.
  • Keeping your body active will help you to keep your digestive system moving, so try to take some regular daily exercise.
  • You may wish to include some foods in your diet that contain sorbitol. Sorbitol is a naturally occurring sugar. It is not digested very well and draws water into your bowel which has an effect of softening stools. Fruits (and their juices) that have a high sorbitol content include apples, apricots, gooseberries, grapes (and raisins), peaches, pears, plums, prunes, raspberries and strawberries.
  • Food such as pastries, puddings, sweets, cheese and cake can make constipation worse and are probably best avoided.
  • You can read more about how to prevent or treat constipation in the separate health information leaflets called Constipation in Adults and Constipation in Children.

Can lactulose cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the ones which can occur with lactulose. You will find a full list in the manufacturer’s information leaflet supplied with your medicine. Speak with your doctor or pharmacist if any of the following continue or become troublesome.

Lactulose side-effects What can I do if I experience this?
Excess wind (flatulence), tummy (abdominal) discomfort or cramps These effects soon settle down as your body adjusts but if they continue or become troublesome, speak with your doctor or pharmacist for further advice
Feeling sick (nausea) or being sick (vomiting) Try taking lactulose with meals, or mixing your dose with some water or fruit juice

If you experience any other symptoms which you think may be due to the medicine, speak with your doctor or pharmacist for further advice.

How to store lactulose

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.

Important information about all medicines

If you buy any medicines check with a pharmacist that they are safe to take with your other medicines.

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

If you are having an operation or dental treatment, tell the person carrying out the treatment which medicines you are taking.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy who will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

What is lactulose?

Lactulose is a synthetic sugar that is an osmotic laxative.

Lactulose may be prescribed by a doctor, but you can also buy it over-the-counter from pharmacies.

It is only available as an oral solution. Duphalac is a well-known brand name for lactulose.

What is lactulose used for?

  • Treating constipation.
  • In higher doses, lactulose is also prescribed by doctors to people with severe liver disease (hepatic encephalopathy and hepatic coma).

How does lactulose work?

Lactulose works in the bowel and is not absorbed into the bloodstream.

Lactulose relieves constipation by causing water to be drawn into the lower bowel. This increases the amount of water in the stools, which softens them and makes them easier to pass. It can take up to 48 hours for the lactulose to have an effect.

In liver disease lactulose works by reducing the absorption of ammonia from the gut. The liver normally breaks down ammonia, but in liver disease such as cirrhosis the blood may bypass the liver, allowing this poisonous substance to pass to the brain. Here it can impair brain function, causing confusion, drowsiness and finally coma. By reducing the absorption of ammonia from the gut, lactulose helps prevent this complication of liver disease.

Key facts about lactulose

  • Lactulose is not a fast-acting laxative, it takes at least 48 hours to work.
  • Drinking plenty of water (6-8 glasses/day) while taking lactulose to help to work more effectively.
  • Common side effects of lactulose include wind and bloating.
  • Lactulose is suitable for children and adults. Only use lactulose for children after consulting your pharmacist or doctor.
  • Lactulose should not be used without medical advice if you are lactose intolerant.

Lactulose: dosage and how do I take

▪️ Follow the instructions given by your doctor or pharmacist regarding how much lactulose to take and how often to take it. The lactulose dose needed will vary from person to person depending on age and condition being treated.

▪️ To treat constipation the dosage for adults is usually 15ml twice a day.

▪️ For children with constipation the dosage depends on their age:

  • Babies from 4 weeks old to 1 year old the usual dose is 2.5ml twice a day.
  • Children aged from 1 to 4 years the usual dose is 5ml twice a day.
  • Children aged from 5 to 17 years the usual dose varies from 5ml to 20ml twice a day.

▪️ Lactulose solution can be taken either with or without food. However, if you find it makes you feel sick this can be reduced by taking it with water, fruit juice or meals. You can take lactulose undiluted, or you can mix the required lactulose dose into a drink of water or fruit juice.

▪️ Missed dose: If you forget to take a dose of lactulose don’t worry, just leave out the missed dose and take the next dose as usual when it is due. Do not take a double dose to make up for a missed dose.

Who should not take lactulose?

  • People with a blockage in their gut (intestinal obstruction).
  • People with a tear or abnormal hole in their gut (perforation).
  • People with an inherited inability to metabolise galactose in the diet (galactosaemia).

Is it safe to take lactulose if pregnant or breastfeeding?

Yes, lactulose isn’t absorbed into the bloodstream and is not known to be harmful if used during pregnancy or breastfeeding.

Side effects of lactulose?

Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects known to be associated with lactulose. Just because a side effect is stated here doesn’t mean that all people using lactulose will experience that or any side effect.

  • Excess gas in the stomach and intestines (flatulence). This usually improves after the first few days of taking lactulose.
  • Feeling sick and vomiting. This can usually be avoided by taking the lactulose with water, fruit juice or a meal.
  • Abdominal cramps and diarrhoea. These usually only occur if the dose is too high.
  • Disturbance in the levels of electrolytes such as sodium or potassium in your blood may occur if the lactulose causes diarrhoea. This is unlikely to happen unless you are taking high doses for an extended period of time to treat hepatic encephalopathy.

If you want any more information about the possible side effects of lactulose, read the leaflet provided with the medicine. You can find a copy of this here

If you think you have experienced side-effects you can report them using the yellow card scheme.

Can I take lactulose with other medicines?

Yes – lactulose is not known to affect other medicines.

Last updated 27.06.2019

Rita Ghelani (BPharm, MRPharmS) Pharmacist A UK registered practising pharmacist with over 20 years’ experience, Rita is a member of the medical journalists’ association (MJA) and has a wealth of experience in community pharmacy.

Lactulose for constipation

This leaflet is about the use of lactulose for constipation.

This leaflet has been written for parents and carers about how to use this medicine in children. Our information sometimes differs from that provided by the manufacturers, because their information is usually aimed at adult patients. Please read this leaflet carefully. Keep it somewhere safe so that you can read it again.

Name of drug

Brand names: Duphalac®, Lactugal®, Laevolac®

Why is it important for my child to take this medicine?

Constipation (difficulty doing a poo) can make your child feel poorly. Lactulose is a type of laxative that works by softening the stool (poo). It should help your child to produce a comfortable soft stool that is easy to pass.

What is lactulose available as?

  • Liquid medicine: 3.1–3.7 g in 5 mL

When should I give lactulose?

Lactulose is usually given twice each day, once in the morning and once in the evening. Ideally, these times are 10–12 hours apart, for example some time between 7 and 8 am, and between 7 and 8 pm.

Give the medicine at about the same times each day so that this becomes part of your child’s daily routine, which will help you to remember.

How much should I give?

Your doctor will work out the amount of lactulose (the dose) that is right for your child. The dose will be shown on the medicine label. It may need to be increased until your child passes a soft stool every day. Your doctor will advise you.

It is important that you follow your doctor’s instructions about how much to give.

How should I give it?

Liquid medicine: Measure out the right amount using a medicine spoon or oral syringe. You can get these from your pharmacist. Do not use a kitchen teaspoon as it will not give the right amount.

When should the medicine start working?

It may take a day or two before lactulose softens the stool enough for your child to pass it more comfortably. Continue to give lactulose to your child during this time. If there is no change after 2-3 days, contact your doctor for advice as you may need to try a different medicine or a higher dose of lactulose.

What if my child is sick (vomits)?

  • If your child is sick less than 30 minutes after having a dose of lactulose, give them the same dose again.
  • If your child is sick more than 30 minutes after having a dose of lactulose, you do not need to give them another dose. Wait until the next normal dose.

If your child is sick again, seek advice from your GP, pharmacist or hospital. They will decide what to do based on your child’s condition and the specific medicine involved.

What if I forget to give it?

If you remember up to 4 hours after you should have given a dose, give your child the missed dose. For example, if you usually give a dose at about 7 am, you can give the missed dose at any time up to 11 am. If you remember after that time, do not give the missed dose. Just give the next dose as usual.

What if I give too much?

It is unlikely that you will cause any harm if you give your child too much lactulose by mistake. If you are concerned, contact your doctor or local NHS services (111 in England and Scotland; 0845 4647 in Wales).

Have the medicine bottle with you if you telephone for advice.

Are there any possible side-effects?

We use medicines to make our children better, but sometimes they have other effects that we don’t want (side-effects).

  • When your child first starts taking lactulose they may have flatulence (passing wind), stomach pain and may feel sick (nausea) or be sick (vomit). These effects usually wear off. If they are a problem after one week, contact your doctor.

There may, sometimes, be other side-effects that are not listed above. If you notice anything unusual and are concerned, contact your doctor. You can report any suspected side-effects to a UK safety scheme at

Can other medicines be given at the same time as lactulose?

  • You can give your child medicines that contain paracetamol or ibuprofen, unless your doctor has told you not to.
  • Check with your doctor or pharmacist before giving any other medicines to your child. This includes herbal or complementary medicines.

Is there anything else I need to know about this medicine?

  • Long-term use of lactulose is very safe. It does not mean your child will always need to take it to open their bowels (do a poo).
  • You can help your child’s constipation by giving them high-fibre foods such as fruit, vegetables, bran and high-fibre cereals to eat. Also encourage them to drink plenty of water, which will help to soften the stools. Encouraging them to be active will also help their constipation. Your pharmacist, doctor or health visitor will be able to give you advice and support.

General advice about medicines

  • Try to give medicines at about the same times each day, to help you remember.
  • If you are not sure a medicine is working, contact your doctor but continue to give the medicine as usual in the meantime. Do not give extra doses, as you may do harm.
  • Only give this medicine to your child. Never give it to anyone else, even if their condition appears to be the same, as this could do harm.
  • If you think someone else may have taken the medicine by accident, contact your doctor for advice.
  • Make sure that you always have enough medicine. Order a new prescription at least 2 weeks before you will run out.
  • Make sure that the medicine you have at home has not reached the ‘best before’ or ‘use by’ date on the packaging. Give old medicines to your pharmacist to dispose of.

Where should I keep this medicine?

  • Keep the medicine in a cupboard, away from heat and direct sunlight. It does not need to be kept in the fridge.
  • Make sure that children cannot see or reach the medicine.
  • Keep the medicine in the container it came in.

Who to contact for more information

Your doctor, pharmacist or nurse will be able to give you more information about lactulose and about other medicines and treatments for constipation.

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