Lactulose: osmotic laxative and liver use explained

Lactulose is a sugar-like substance that the body does not absorb. It is used in two main settings: to treat constipation, meaning infrequent or hard stools, and to treat hepatic encephalopathy, a condition in which liver disease allows toxins to affect the brain.

It is available only as a liquid you swallow. Adults and children aged 14 and over can buy it without a prescription from pharmacies, while younger children need a doctor to prescribe it.

Because lactulose stays in the bowel, it brings water in with it, which makes stools softer and easier to push out. This is called an osmotic effect. It is gentle rather than fast: the NHS says it can take 2 to 3 days to start working, so there is no point increasing the dose after a day.

If constipation has not eased after 3 days, or is getting worse, see a pharmacist or doctor.

The NHS does not give a standard dose for lactulose; the amount and how long you take it depend on the reason and on your pharmacist or doctor's advice. Measure with the spoon or cup in the box, take doses evenly spaced through the day, and take it with or without food. Mixing it with squash can make the taste easier.

Drink plenty of fluids while taking it. Staying hydrated is part of how it relieves constipation as well as protecting against dehydration if stools become loose.

• If you miss a dose, leave it and carry on with your usual schedule.

• Do not double up to make up for a missed dose.

• An extra dose is unlikely to cause harm, but call NHS 111 if you have vomiting, diarrhoea, bloating, dizziness or weakness after taking too much.

Lactulose is not suitable if you have had an allergic reaction or serious side effect from it or a similar medicine, if you are intolerant of lactose or allergic to galactose or fructose, or if you have the rare inherited condition galactosaemia. A bowel blockage that is not caused by constipation also rules it out.

The NHS lists irritable bowel syndrome, diverticular disease and Crohn's disease as conditions to check about first. People with diabetes who take lactulose for hepatic encephalopathy need extra care, since high doses can change blood sugar.

Wind is the best-known effect and should ease after a few days. Bloating, stomach ache, nausea, vomiting and diarrhoea can also occur. If they bother you or do not go away, talk to a pharmacist or doctor.

Rare, serious effects include allergic reaction, for which you should call 999 if you have a swollen throat or tongue, an itchy rash or trouble breathing, and low blood potassium after prolonged heavy diarrhoea, which can cause muscle weakness, tiredness, passing urine often and heart rhythm changes. Call NHS 111 about other suspected serious effects. Suspected side effects can be reported to the Yellow Card scheme.

You can take lactulose in pregnancy, but it is best to speak to a pharmacist or doctor first. It can be taken while breastfeeding, and side effects in a baby are uncommon. The NHS pages do not comment on fertility.

The NHS says some medicines, remedies and supplements may not mix well with lactulose but does not name any. Tell your pharmacist everything you take, including herbal products.

You can eat and drink as normal while using lactulose, and the NHS does not set an alcohol restriction. It does not usually affect driving, cycling or using machinery.

Long-term high-dose use can cause low potassium, particularly if diarrhoea becomes frequent, so any ongoing use should be reviewed with a clinician.