Ispaghula husk (Fybogel): bulk-forming laxative

Ispaghula husk is a natural fibre from the seed husks of a plant. Fybogel is the best-known brand, with Ispagel also named by the NHS. As a bulk-forming laxative it works much as dietary fibre does, by holding fluid in the stool so that it is bigger and softer and easier to pass.

It is available on prescription and from pharmacies and shops, as plain or fruit-flavoured granules.

Empty the granules into a glass with at least 150 ml of cold water, stir until dissolved and the solution is clear or slightly hazy, and drink it straight away. Avoid breathing in the powder, which can cause a mild allergic reaction.

For adults and children aged 12 and over the NHS gives 1 sachet twice a day. Children aged 6 to 11 take half to one level 5 ml teaspoonful twice a day, and under-6s only if a doctor recommends it. Take it after a meal or snack, and the evening dose at least an hour before bed.

• It usually takes 2 to 3 days to work; see a doctor if there is no improvement after 3 days.

• Skip a missed dose and take the next as normal.

• An overdose may cause wind and stomach pain; drink water and contact NHS 111 if concerned.

• More serious cases may need treatment for several months, under medical guidance.

A fibre drink without enough overall fluid can backfire and make constipation worse. The NHS advises adults to try to drink more than 2 litres, around 8 glasses, of fluid a day while taking it. Wheat bran and extra fibre in food can be added as part of a longer-term plan, with the NHS suggesting about 30 g of fibre a day, along with regular activity.

Most adults can take Fybogel, including during pregnancy and breastfeeding, and children under 6 need a prescription. Tell a pharmacist or doctor if you have ever reacted to it, if you have unusual changes in your bowel habit or blood in the stool lasting more than 2 weeks, or if you have tummy pain with nausea or vomiting, which can signal a blockage.

Weakness of the bowel muscle (colonic atony), swallowing difficulties or throat problems, and the rare inherited condition phenylketonuria are also reasons to check.

Wind and bloating are the main effects and tend to ease within days as the body adjusts. An itchy eye, nose or skin suggests a mild allergy: stop and speak to a pharmacist or doctor.

Rarely, anaphylaxis can occur. Call 999 for swelling of the lips, mouth, throat or tongue, fast or difficult breathing, tight throat or collapse, and do not drive yourself. Suspected effects can be reported to the Yellow Card scheme.

Fybogel is commonly taken in pregnancy and is not harmful to the baby. It does not pass into breast milk and has not caused effects in breastfed babies, and there is no evidence it reduces fertility.

Because it can affect absorption of other medicines, vitamins and supplements, leave at least an hour around other doses, and the NHS flags lithium, carbamazepine, digoxin, insulin and other diabetes medicines, mesalazine and codeine in particular. It can be taken with IBS remedies such as Buscopan, but other laxatives only with advice.

Alcohol is fine, and Fybogel does not affect the ability to drive or ride a bike. Refined foods such as pastries, ice cream, cheese and meat can make constipation worse, so the diet around the medicine matters as much as the sachet.