Bisacodyl (Dulcolax): tablets and suppositories

Bisacodyl is a laxative that helps the bowel move and also softens stools. It is used for constipation, and in hospital it is given to empty the bowel before surgery, examinations or treatments.

It is available both on prescription and over the counter, though over-the-counter packs are small: up to 20 standard tablets or 10 maximum-strength tablets. The brand most people know is Dulcolax.

The two forms behave differently. Tablets work in 6 to 12 hours, so they are taken once a day just before bed to produce a movement the next morning. Suppositories are placed in the back passage and usually work in 10 to 45 minutes, which means staying close to a toilet.

Adults and young people aged 12 and over take 5 mg to 10 mg as a bedtime tablet, starting low and going to 10 mg if needed. The NHS gives one 10 mg suppository for adults and children aged 10 and over, and a 5 mg suppository for ages 4 to 9 with a doctor's approval. Your pack or prescriber has the final say.

• Swallow tablets whole with water and not with milk.

• Leave an hour between bisacodyl and dairy products, indigestion remedies or acid reducers.

• Skip a missed dose and take the next one the following evening, never doubling up.

• An extra dose is unlikely to harm, though it may cause diarrhoea and stomach pain.

The NHS says not to take tablets or use suppositories every day for more than 5 days. Longer use risks dependence, and the bowel may start to rely on the stimulant. In the long term it can also cause diarrhoea and a disturbance of body salts, leading to muscle spasms, twitching or seizures. If constipation continues, speak to a pharmacist or doctor rather than extending the course.

Adults aged 18 and over can use both forms. Young people aged 12 to 17 can if a doctor or pharmacist agrees, and children aged 11 and under only on prescription.

Talk to a pharmacist or doctor first if you have had a reaction to bisacodyl, show signs of dehydration, have severe abdominal pain with nausea or vomiting, appendicitis or a bowel obstruction, ulcerative colitis or Crohn's disease, or weak bowel muscles. For suppositories, mention anal fissures or cracked skin around the anus.

Nausea, diarrhoea and stomach pain or cramps are the common ones. Taking it with a little food can help nausea. If diarrhoea develops, stop the medicine and drink more fluids; severe diarrhoea lasting more than 24 hours may make the contraceptive pill less effective. Cramps may ease with a lower dose, rest, smaller meals and a heat pad.

Call NHS 111 for dizziness, blood in the stool or vomiting. Call 999 for swelling of the lips, mouth, throat or tongue, severe breathing difficulty, or collapse. Suspected side effects can be reported through the Yellow Card scheme.

Bisacodyl is not generally recommended in pregnancy; constipation is usually first managed with diet and lifestyle, then lactulose or Fybogel, and a doctor should guide anything further. It is acceptable during breastfeeding when non-drug measures fail, as it is not thought to pass into milk, and should be used for the shortest time possible. The NHS says there is no evidence it reduces fertility.

Diuretics, steroids such as prednisolone and digoxin can be affected by the salt imbalances that heavy use of bisacodyl may cause. Antacids stop the tablets working properly and can strip their protective coating, so leave an hour either side. Do not combine laxatives unless a clinician advises.

The NHS says you can drink alcohol with bisacodyl and that it does not stop you driving or riding a bike. Alongside it, aim for around 30 g of fibre a day, drink enough, stay active and answer the urge to go without delay. Pastries, cakes, puddings and cheese tend to make constipation worse.