Senna

Senna is a plant-derived stimulant laxative whose active components are sennosides A and B (anthraquinone glycosides). Colonic bacteria convert them to rhein anthrone, which increases colonic motility and secretion. It is a first-line stimulant laxative for opioid-induced constipation and everyday constipation.

Classification. Stimulant laxative; anthraquinone.

Mechanism. The glycosides pass unchanged through the small intestine and are hydrolysed in the colon to rhein anthrone, which stimulates enteric neurons, increases peristalsis and inhibits water absorption.

Pharmacodynamics. Laxative effect begins 6 to 12 hours after oral dosing, so doses are given at bedtime. Tolerance to regular use is uncommon at usual doses.

Pharmacokinetics. Minimal absorption of the glycosides. The active metabolite is partly absorbed and excreted in urine (colouring it yellow-brown or red) and in milk in small amounts.

Adult oral doses unless stated. Doses are given as sennosides.

Constipation.

• Sennosides 7.5 to 15 mg once daily at bedtime, increased to 30 mg at bedtime if needed.

• Maximum 30 mg daily for labelled use.

• Palliative care specialists may use up to 30 mg twice daily.

• Use for the shortest period needed; seek review if used for more than 1 week without effect.

Opioid-induced constipation. Sennosides 15 to 30 mg at bedtime, started when the opioid is started, with an osmotic laxative (macrogol or lactulose) added if stool remains hard.

Bowel preparation before surgery or colonoscopy. Sennosides 15 to 30 mg the day before, as part of a regimen directed by the proceduralist.

Paediatric dosing (stimulant laxative, usually second-line).

• Age 2 to 5 years: 3.75 to 7.5 mg at bedtime.

• Age 6 to 11 years: 7.5 to 15 mg at bedtime.

• Age 12 years and over: 15 to 30 mg at bedtime.

Kidney and liver adjustment. No adjustment. Use caution in cirrhosis because of volume loss.

Contraindications. Intestinal obstruction or ileus; acute abdomen (appendicitis); acute inflammatory bowel disease; toxic megacolon; severe dehydration; undiagnosed abdominal pain; hypersensitivity.

Common adverse effects. Abdominal cramp, diarrhoea, nausea and yellow-brown or red discolouration of urine (harmless).

Serious adverse effects.

• Dehydration and hypokalaemia with overuse or diarrhoea; arrhythmia in patients on digoxin.

• Melanosis coli (benign brown pigmentation of the colon) after prolonged use.

• Rare hepatotoxicity with chronic use of herbal senna in high doses.

• Clubbing of fingers with chronic abuse.

Major interactions.

• Digoxin: hypokalaemia increases toxicity.

• Diuretics and corticosteroids: additive potassium loss.

• Other drugs prolonging the QT interval: hypokalaemia increases arrhythmia risk.

• Oral drugs may be less well absorbed from faster transit.

Special groups.

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

• Breastfeeding: minimal excretion; compatible.

• Children: second-line; lower doses by age.

• Older adults: monitor electrolytes and avoid long-term use; first-line stimulant in opioid use.

• Kidney impairment: monitor electrolytes.

• Liver impairment: use with caution.

• Regional: widely available; herbal senna teas and powders may vary in strength.

Formulations. Tablets 7.5 mg sennosides; oral syrup 7.5 mg in 5 mL; granules; herbal teas.

Administration. Take at bedtime with a glass of water. The onset is 6 to 12 hours.

Baseline. Cause of constipation, and exclusion of obstruction and alarm features. Review the opioid, anticholinergic and calcium-channel blocker burden.

Monitoring.

• Stool frequency, abdominal pain and diarrhoea.

• Potassium and kidney function with prolonged high-dose use or interacting drugs.

• Hydration in older adults.

Therapeutic drug monitoring. Not applicable.

Stopping. Reduce or stop for diarrhoea or cramps. 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. Urgent review for severe pain, distension or vomiting.