Macrogol (Movicol, Laxido): laxative for constipation and impaction

Macrogol is a long-chain compound that the gut does not absorb. Because it stays in the bowel, it keeps water there, which makes stools softer and easier to pass. That is why the NHS classes it as an osmotic laxative.

The NHS lists three uses: treating constipation, treating faecal impaction (hard stool that has built up and cannot be passed), and emptying the bowel ahead of tests such as a colonoscopy.

Brand names are Movicol, Laxido and CosmoCol. Which brand or strength you are given depends on the reason for use.

Macrogol comes as a powder that you dissolve in water or as a ready-made liquid. How much you take, and for how long, depends on the type and on why you are taking it, so follow the label, the leaflet or the prescriber rather than a general figure.

Drinking plenty of fluid is part of treatment. You can take macrogol with or without food, and you can add a flavouring if you like.

• Leave at least an hour before or after taking other medicines, since fast-moving bowel contents can carry them past the point of absorption.

• If your next dose is nearly due, skip the one you missed and carry on as usual.

• If you have taken too much and have vomiting, diarrhoea, bloating, dizziness or weakness, contact NHS 111.

• See a doctor if constipation has not improved after 2 weeks.

The NHS says most adults and children aged 12 and over can take macrogol. Children from 2 to 11 can only have it on prescription.

It is not suitable if you have had an allergic reaction to it, and the NHS asks you to check first if you have a bowel condition such as diverticular disease or Crohn's disease, kidney or heart conditions, or have been advised to limit potassium or sodium in your diet, because some products contain both.

Stomach ache, bloating, wind, nausea, vomiting and diarrhoea are the common effects. They may improve if the dose is reduced, which is something to agree with a pharmacist or prescriber.

Rarer serious problems include anaphylaxis, swelling from fluid build-up (oedema) and a raised blood potassium level (hyperkalaemia). Call NHS 111 about serious symptoms, and 999 if you have signs of a severe allergic reaction such as a swollen throat or tongue or difficulty breathing.

Suspected side effects can be reported through the Yellow Card scheme.

The NHS says macrogol can be taken during pregnancy and while breastfeeding.

The one medicine group the NHS singles out is epilepsy treatment: macrogol can affect how some medicines for epilepsy work. If you take anti-seizure medication, tell your prescriber or pharmacist before starting it and keep to the one-hour gap.

Macrogol does not usually affect the ability to drive, ride a bike or use machinery. You can eat and drink as normal, though enough fluid is essential, especially if you have loose stools, to prevent dehydration.

Other osmotic laxatives include lactulose, while bulk-forming options such as ispaghula husk and stimulants such as senna and bisacodyl work differently and have their own guides. A pharmacist can help match the type to the problem. Prescription and over-the-counter products both exist, and children aged 2 to 11 are treated under medical supervision.