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Side effects of lactulose

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 of lactulose, particularly at high doses, is diarrhoea. This happens in more than 1 in 10 people.

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 help. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.

If you take contraceptive pills and you have severe diarrhoea, your contraception may not protect you from pregnancy. Check the pill packet for advice.

These other common side effects happen in more than 1 in 100 people. There are things you can do to help cope with them:

Bloating

Take lactulose between meals instead of before or after them.

Wind (farting and burping)

Do not eat 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 simeticone.

Feeling sick (nausea)

Try taking lactulose with meals, or mixing your dose with some water or fruit juice.

Being sick (vomiting)

If you're being sick a lot, drink lots of fluids, such as water or squash, to avoid dehydration. It may help to take small, frequent sips. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.

Do not take any other medicines to treat vomiting without speaking to a pharmacist or doctor. If it does not improve after a couple of days, contact your doctor for advice.

If you take contraceptive pills and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.

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.

These side effects are mild and usually go away after a couple of days. Talk to your doctor or pharmacist if the advice on how to cope does not help and side effects bother you or do not go away.

Serious side effects

It happens rarely (less than 1 in 100 people), but some people have serious side effects when taking lactulose.

Call a doctor or contact 111 straight away if you have severe diarrhoea or vomiting for more than a couple of days and you get:

  • muscle cramps or weakness
  • an irregular heartbeat

This could be a sign of an electrolyte imbalance, where levels of substances like sodium, potassium and magnesium in your body get too high or too low.

Go to 111.nhs.uk or call 111

Serious allergic reaction

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

Immediate action required: Call 999 or go to A&E now 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.

Other side effects

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

Information:

You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

Page last reviewed: 4 March 2022
Next review due: 4 March 2025