senna
stimulant laxative· Brands: Senokot, Ex-Lax, Senna Laxative, Pursennid, Senna SyrupPO
Mechanism of action
Senna is a prodrug. Its active components, sennosides, pass through the upper gastrointestinal tract largely unabsorbed. In the colon, they are hydrolyzed by bacterial glycosidases to the active metabolite rhein anthrone. Rhein anthrone stimulates the myenteric plexus, increasing colonic motility and net secretion of water and electrolytes, which leads to increased faecal volume and a laxative effect. It also inhibits colonic water absorption, contributing to its action.
Pharmacokinetics
The sennosides are poorly absorbed in the small intestine and reach the colon unchanged. There, colonic bacteria convert them to active metabolites, which act locally on the colonic mucosa. Systemic absorption of the active metabolite is minimal, although some may be absorbed. Onset of action is typically 6 to 12 hours after an oral dose, with peak effect occurring within 48 hours. The drug is excreted mainly in the faeces, with small amounts of active and inactive metabolites found in the urine.
Indications & dosing
Constipation
Typically taken at bedtime. Onset 6-12 hours.
- adult · PO: 15–30 mg once daily · Typically taken at bedtime. Onset 6-12 hours.
Constipation (children 6-12 years)
Children 6-12 years. Give at bedtime.
- pediatric · PO: 7.5–15 mg once daily · Children 6-12 years. Give at bedtime.
Contraindications & precautions
- Intestinal obstruction · Stimulant laxatives are contraindicated in bowel obstruction or ileus.
- Acute abdominal pain of unknown origin · Should be investigated before using laxatives.
- Inflammatory bowel disease · Use with caution in active disease; stimulant laxatives may exacerbate symptoms.
- Dehydration or electrolyte imbalance · Correct fluid and electrolyte status before use.
- Pregnancy · Avoid in the first trimester; use only if clearly needed and under medical advice.
- Children under 2 years · Not recommended; safety and efficacy not established.
- Avoid prolonged use; limit to 7 days unless advised by a healthcare professional.
- Use with caution in pregnancy and lactation.
- Ensure adequate hydration, especially when using with diuretics or in the elderly.
- Monitor electrolytes if used regularly, particularly in patients at risk of hypokalaemia.
- Discontinue if severe abdominal pain or bleeding occurs; seek medical attention.
- May cause harmless discolouration of urine; inform patients to avoid alarm.
Adverse effects
- Abdominal cramps
- Diarrhoea
- Nausea
- Electrolyte imbalance, especially hypokalaemiaserious
- Laxative dependence with prolonged useserious
- Melanosis coli
- Allergic reactionsserious
Monitoring, pregnancy & toxicity
- Bowel movement frequency · Prior to and during treatment as needed
- Serum electrolytes (potassium, sodium) · For prolonged use or in patients at risk of electrolyte disturbances
- Hydration status and fluid intake · Assess with prolonged use or in frail/elderly patients
- Pregnancy: Senna is not recommended in pregnancy, particularly during the first trimester, as stimulant laxatives may increase uterine tone and the risk of preterm labour. Non-pharmacological measures or bulk-forming laxatives are preferred.
- Toxicity: Severe diarrhoea and abdominal cramps · Discontinue senna; provide supportive care with rehydration and electrolyte replacement.
- Toxicity: Fluid and electrolyte depletion, especially hypokalaemia · Monitor serum electrolytes and replace potassium as necessary; monitor fluid balance.
- Toxicity: Laxative dependence with chronic misuse · Gradual withdrawal of the laxative and patient education; address underlying constipation causes.
Clinical notes
Senna is used for the short-term treatment of constipation and to facilitate bowel evacuation before diagnostic procedures such as colonoscopy. It is usually taken at night to produce a morning stool. The dose is individualised, starting with the lowest effective dose and titrating as needed. Prolonged use is not recommended, as it can lead to dependence, electrolyte disturbances, especially hypokalaemia, and melanosis coli. It is contraindicated in intestinal obstruction, acute inflammatory bowel disease, and severe dehydration. Cautious use is advised in pregnancy, children, and patients with cardiac or renal impairment. If no bowel movement occurs after 3 to 5 days, medical review is warranted.
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BNF; local formulary