Senna — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Senna?
Senna is a stimulant laxative derived from the Senna plant, used for the treatment of constipation. This medication is available over-the-counter; also funded by Pharmac.
What is Senna Used For?
Senna is used for the short-term treatment of constipation and as part of bowel preparation before procedures. It is commonly used as part of an opioid-induced constipation prevention regimen.
How Does Senna Work?
Sennosides (active glycosides in senna) are hydrolysed by colonic bacteria to active compounds (rheinanthrone) that stimulate peristalsis by acting on myenteric neurons and intestinal smooth muscle — increasing colonic motility and producing a bowel action within 6–12 hours.
How to Take Senna
Taken at bedtime for a bowel action in the morning. Standard dose: 15 mg (one tablet of Senokot®) at bedtime, increased to 30 mg if needed. Maximum 30 mg twice daily. Also available as granules and liquid.
Common Side Effects of Senna
- Abdominal cramping
- Diarrhoea (with excessive doses)
- Nausea
- Discolouration of urine (pink to reddish-brown — harmless)
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- Electrolyte disturbances (hypokalaemia — with excessive chronic use)
- Chronic laxative dependency
- Melanosis coli (benign brown discolouration of colon)
Drug Interactions
Senna may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:
- Diuretics (additive hypokalaemia with chronic overuse)
New Zealand Prescribing Information
Senna (7.5 mg sennosides tablets — Senokot® and generics) is available OTC at NZ pharmacies and funded on prescription. Senna is the recommended stimulant laxative for opioid-induced constipation in NZ palliative care guidelines — patients starting opioids should routinely be prescribed senna (or bisacodyl) alongside the opioid. Do not use in intestinal obstruction, undiagnosed abdominal pain, or inflammatory bowel disease.
Frequently Asked Questions
Can senna be used in pregnancy?
Senna is generally considered safe for short-term use in the second and third trimesters of pregnancy when other measures (dietary fibre, fluids, gentle exercise) have not relieved constipation. It should be avoided in the first trimester. Lactulose or macrogol are often preferred as first-line in pregnancy.
Why do people on opioids need a laxative?
Opioids slow intestinal motility by acting on opioid receptors in the gut, causing constipation in nearly all patients. Unlike other opioid side effects, tolerance to constipation does NOT develop. A stimulant laxative (senna or bisacodyl) should be prescribed routinely whenever opioid therapy is started, and continued throughout treatment.
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.
Reviewed by a Registered Pharmacist NZ
References & Further Information
The following New Zealand and international resources were used to inform this page: