Bisacodyl

Bisacodyl is a diphenylmethane stimulant laxative. It acts on the colonic mucosa to increase peristalsis and fluid secretion, with oral onset after 6 to 12 hours and rectal onset after 15 to 60 minutes. It is used for constipation, faecal impaction and bowel preparation.

Classification. Stimulant laxative.

Mechanism. Deacetylated by mucosal and bacterial esterases to the active desacetyl metabolite, which stimulates enteric nerves, increases colonic motility, and reduces water absorption while promoting water and electrolyte secretion.

Pharmacodynamics. Oral enteric-coated tablets act in the colon, 6 to 12 hours after the dose. Suppositories act on the rectum within 15 to 60 minutes.

Pharmacokinetics. Less than 5% of the active metabolite is absorbed and is excreted in urine as glucuronide. Most is excreted in faeces.

Adult doses unless stated.

Constipation.

• Oral: 5 to 10 mg once daily at bedtime (enteric-coated tablet), increased to 15 mg if needed.

• Rectal: 10 mg suppository once daily in the morning.

• Maximum oral 20 mg daily for constipation.

• Use short courses; seek review if needed for more than 1 week.

Opioid-induced constipation. 10 mg orally at bedtime, increased to 15 to 20 mg; or 10 mg suppository daily if oral therapy fails. Combine with an osmotic laxative if stool is hard.

Faecal impaction. 10 mg suppository once daily after soft stool achieved with osmotic laxatives; or bisacodyl with macrogol under supervision.

Bowel preparation before colonoscopy. 10 to 20 mg orally the evening before, usually in addition to an osmotic preparation; a 10 mg suppository on the morning of the procedure is added in some regimens.

Paediatric dosing.

• Oral: age 4 to 9 years 5 mg at bedtime; age 10 to 17 years 5 to 10 mg at bedtime (up to 20 mg).

• Rectal: age 2 to 9 years 5 mg; age 10 years and over 10 mg.

• Under 4 years oral and under 2 years rectal: specialist use only.

Kidney and liver adjustment. No adjustment.

Contraindications. Intestinal obstruction; acute abdominal conditions (appendicitis, acute inflammatory bowel disease); severe dehydration; hypersensitivity. Avoid suppositories in anal fissure with pain and in proctitis.

Common adverse effects. Abdominal cramp, diarrhoea, nausea and rectal irritation with suppositories.

Serious adverse effects.

• Dehydration, hypokalaemia and hypomagnesaemia with overuse.

• Ischaemic colitis and, rarely, colonic perforation after bowel preparation.

• Rectal mucosal injury or proctitis with repeated suppository use.

• Syncope with severe cramp.

Major interactions.

• Antacids, PPIs, H2 receptor antagonists and milk: dissolve the enteric coating early and cause gastric irritation; separate by at least 1 hour.

• Diuretics and corticosteroids: additive potassium loss.

• Digoxin: hypokalaemia increases toxicity.

• Other laxatives: additive effect.

Special groups.

• Pregnancy: acceptable for short use when bulk-forming or osmotic laxatives fail.

• Breastfeeding: compatible.

• Children: second-line, short use.

• Older adults: monitor electrolytes and avoid long-term use.

• Kidney impairment: monitor electrolytes.

• Liver impairment: use with caution.

Formulations. Enteric-coated tablets 5 mg; suppositories 5 mg and 10 mg.

Administration. Swallow tablets whole with water; do not crush or chew. Do not take within 1 hour of antacids, PPIs or milk. Insert suppositories into the rectum with moistened tip and retain for 15 to 20 minutes.

Baseline. Cause of constipation and exclusion of obstruction.

Monitoring.

• Bowel pattern, pain and diarrhoea.

• Potassium with prolonged use or interacting drugs.

• Hydration during bowel preparation.

Therapeutic drug monitoring. Not applicable.

Stopping. Reduce or stop for cramps or diarrhoea. No taper is needed.

Referral and escalation. Colorectal referral for red flags: weight loss, rectal bleeding, iron deficiency anaemia, or change in bowel habit over age 50. Imaging for suspected obstruction.