Senna
Senna alexandrina (Cassia senna, Cassia angustifolia)
A stimulant laxative herb whose sennosides reliably produce a bowel movement in 6–12 hours; it is FDA-approved and in the WHO essential medicines list for short-term constipation, but regular use causes dependence, cramping, and electrolyte loss.
At a Glance
- Best known for
- Short-term relief of occasional constipation (well-established), Bowel preparation before colonoscopy or surgery, Opioid-induced constipation (often with a stool softener)
- Evidence
- Strong
- Typical dose
- 15–30 mg sennosides once daily at bedtime
- Type
- Herb
What is Senna?
Senna is a shrub native to the Arabian Peninsula, Egypt, Sudan, and India whose leaves and seed pods have been used as a laxative since at least the 9th century, when Arab physicians introduced it to European medicine. It is the active ingredient in many over-the-counter laxatives (such as Senokot and Ex-Lax) and is one of the few herbal medicines that is simultaneously an FDA-approved OTC drug, a WHO essential medicine, and a common ingredient in 'detox' and 'slimming' teas. The German Commission E and the European Medicines Agency approve senna for short-term use in occasional constipation. Its reliability is well established; the concerns are around dependence and misuse rather than efficacy.
Known Health Benefits
How It Works
Sennosides are inactive in their glycoside form and pass through the stomach and small intestine unchanged. In the colon, gut bacteria cleave off the sugar and reduce them to the active compound rhein anthrone. Rhein anthrone acts in two ways: it directly stimulates the myenteric nerve plexus in the colon wall, increasing peristaltic contractions and speeding transit, and it alters fluid and electrolyte transport across the colonic lining, inhibiting water and sodium absorption while increasing secretion of water into the lumen. The combined effect is a softer, larger stool propelled through the colon more quickly, usually within 6–12 hours. Because the activation step requires colonic bacteria, senna does not act in the small intestine and has no effect for several hours after dosing.
What Research Says
Senna's efficacy is well documented. Systematic reviews of stimulant laxatives, including a 2010 American College of Gastroenterology review and a 2011 Cochrane review of laxatives in palliative care, consistently show senna increases stool frequency and reduces straining compared with placebo, with an effect comparable to other stimulant laxatives and, in some trials, to polyethylene glycol. In bowel preparation for colonoscopy, senna combined with polyethylene glycol or magnesium citrate improves cleansing and tolerability in several RCTs. A 2007 RCT in opioid-treated cancer patients found senna as effective as lactulose and cheaper. Controlled trials of senna for chronic constipation beyond 4 weeks are scarce, and most guidelines recommend it for occasional or rescue use because of concerns about tolerance, cramping, and electrolyte disturbance. Earlier fears that long-term senna use causes colon cancer or permanent nerve damage ('cathartic colon') have not been supported by modern evidence; melanosis coli is reversible and benign. Case reports link chronic high-dose senna to liver injury and to hypokalemia with cardiac effects. The evidence tier is strong for short-term constipation relief.
Active Compounds
Sennosides A and B (dianthrone glycosides; 1.5–3% in leaf, higher in pods), rhein, aloe-emodin, flavonoids, mucilage
Forms & Bioavailability
Sennosides are poorly absorbed in the upper gut, which is desirable: they travel to the colon, where bacterial enzymes convert them into the active rhein anthrone. Onset is typically 6–12 hours after an oral dose, so bedtime dosing produces a morning bowel movement. A small amount of rhein is absorbed and excreted in urine (coloring it yellow-brown or reddish) and in breast milk in trace amounts. Standardized tablets give consistent dosing, whereas leaf and pod teas vary widely in sennoside content depending on the plant part, steeping time, and brand, and are easy to over-brew. Antibiotics that reduce colonic bacteria can diminish senna's effect.
Dosage Guidance
| Use Case | Dosage |
|---|---|
| Occasional constipation (adults, 12+) | 15–30 mg sennosides once daily at bedtime |
| Children 6–11 | 7.5–15 mg sennosides once daily |
| Children 2–5 | 3.75–7.5 mg sennosides once daily |
| Opioid-induced constipation | 15–30 mg sennosides 1–2x daily, often with docusate |
| Bowel preparation | As directed in the prescribed regimen (often 24–36 mg or more) |
Always consult a healthcare provider for personalized dosing.
Potential Side Effects
Abdominal cramps, diarrhea, nausea, urgency; with regular use: low potassium, dehydration, laxative dependence, melanosis coli (harmless brown pigmentation of the colon), reddish-brown urine; rare liver injury with prolonged high doses
Who Should Avoid It
- Intestinal obstruction, ileus, or severe abdominal pain of unknown cause (risk of perforation; needs evaluation)
- Inflammatory bowel disease flare (Crohn's, ulcerative colitis) or appendicitis
- Severe dehydration or electrolyte imbalance, especially low potassium
- Chronic daily use for weight loss or 'detox' (dependence, electrolyte loss; does not reduce calorie absorption meaningfully)
- Children under 2 years
- Heart disease treated with digoxin or antiarrhythmics without potassium monitoring
Pregnancy & Lactation
Senna is considered acceptable for short-term use in pregnancy when bulk-forming fibers, fluids, and stool softeners have failed, and it is not associated with birth defects; however, it can cause cramping and is generally reserved for occasional use, with osmotic laxatives (polyethylene glycol, lactulose) usually preferred. It should be avoided in late pregnancy if there is any concern about preterm labor. During breastfeeding, only trace amounts of rhein pass into milk, and standard doses have not been shown to cause diarrhea in nursing infants; it is commonly used after childbirth.
Known Drug Interactions
Low potassium from senna increases toxicity of digoxin and antiarrhythmics; additive potassium loss with diuretics and corticosteroids; may reduce absorption of other oral drugs by speeding transit; may enhance warfarin effect through diarrhea; avoid with other stimulant laxatives
Evidence Classification
Supported by randomized controlled trials (RCTs), systematic reviews, or meta-analyses published in peer-reviewed journals.
Frequently Asked Questions
How long does senna take to work, and when should I take it?
Senna usually produces a bowel movement 6–12 hours after a dose, because it must reach the colon and be activated by bacteria. Taking it at bedtime typically gives a morning result. It will not work within an hour or two like a suppository or enema. If nothing happens after 12 hours, do not double the dose; try again the next night and consult a clinician if constipation persists beyond a week.
Is it safe to take senna every day?
Not routinely. Daily use for more than a week or two can lead to tolerance (needing larger doses), dependence on it for any bowel movement, cramping, dehydration, and low potassium, which can affect the heart. Older fears that it permanently damages the colon are not well supported, but the practical problems are real. For chronic constipation, bulk fibers, osmotic laxatives like polyethylene glycol, adequate fluids, and exercise are preferred, with senna kept for rescue use. People on opioids or in palliative care may use it longer under supervision.
Do senna 'detox' or 'skinny' teas help with weight loss?
No. Senna empties the colon of stool and water, which can briefly lower the number on the scale, but it does not reduce the absorption of calories or fat. The weight returns as soon as you rehydrate. Regular use of these teas causes the same dependence and electrolyte problems as any stimulant laxative overuse, and they are not regulated for sennoside content, so dosing is unpredictable.
What is melanosis coli, and should I worry about it?
Melanosis coli is a brown-black pigmentation of the colon lining seen on colonoscopy in people who use anthraquinone laxatives like senna regularly. It is caused by pigment-laden cells and is harmless and reversible, typically fading within a year of stopping. It was once thought to signal cancer risk, but studies have not found a link. It mainly serves as a sign to a gastroenterologist that stimulant laxatives are being used frequently.
References
- Senna. NHS (2022)View study
- Senna. MedlinePlus (2024)View study
- Laxatives for the management of constipation in people receiving palliative care. Candy B, Jones L, Larkin PJ, et al.. Cochrane Database of Systematic Reviews (2015)
- Constipation. NHS (2023)View study
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This entry is for educational purposes only. It is not medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement regimen, especially if you take medications or have health conditions.