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 the combined pill or progestogen-only pill and you have severe diarrhoea for more than 24 hours, 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 the combined pill or progestogen-only pill 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 call 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.
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to lactulose.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of 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.
You can report any suspected side effect using the Yellow Card safety scheme.