Senna

Senna alexandrina Mill. belongs to Fabaceae (Leguminosae), the legume or pea family, and NCBI Taxonomy files it as taxon 72402, Alexandrian senna. Philip Miller named it in the eighth edition of his Gardeners Dictionary in 1768 (World Flora Online). Medicine knew it for centuries under older names: Cassia senna L. (1753), Cassia angustifolia Vahl (1790), and Cassia acutifolia Delile (1813), which is why many labels and papers still say Cassia. PROTA (Plant Resources of Tropical Africa) explains that until the early 1980s Cassia was a huge genus of about 550 species, which botanists then split into Cassia in the strict sense (about 30 species), Chamaecrista (about 250), and Senna (about 270); Senna is told apart by three short, straight upper stamens and flower stalks without small bracts. The trade long distinguished Alexandrian or Khartoum senna from the Nile valley and Tinnevelly senna from southern India, but EMA now treats both as Senna alexandrina, and PROTA notes that the trade distinction has no taxonomic basis. Two varieties are recognized: var. obtusata in Eritrea, Ethiopia, Somalia, and northern Kenya, and the widespread var. alexandrina.

PROTA describes a deciduous shrub up to 3 m tall. Each leaf is pinnate, with 4 to 10 pairs of lance-shaped to narrowly elliptical leaflets, 2–6.5 cm long and 0.5–1.5 cm wide, with a lopsided base and short hairs on both sides. Upright flower clusters 5–30 cm long carry 20 to 30 flowers with five yellow or orange-yellow petals, 1.5–3 cm long, and ten stamens of three sizes, the three shortest sterile. The fruit is a flat, slightly curved, oblong pod 4–7 cm by 1.5–2.5 cm, divided crosswise into chambers holding 9 to 16 seeds. The plant flowers and fruits all year and, unusually for a legume, does not form root nodules or fix nitrogen. World Flora Online gives its native range as the grasslands and deserts of tropical Africa, mostly north of the equator, extending east through the Near East to the plains of peninsular India; it is naturalized in Hispaniola and southern Mexico. PROTA places it in semi-desert scrub, floodplains, and riverbanks, often with acacias, from sea level to 1,300 m; older plants tolerate salt but not waterlogging.

India is the world’s largest producer and exporter of senna leaves, pods, and purified sennosides. A 2015 bulletin from the Indian Council of Agricultural Research (ICAR) describes it as a rainfed or lightly irrigated crop of semi-arid Tamil Nadu, Rajasthan, and Gujarat, where irrigated fields are first picked about 90 days after sowing and again at 150 and 210 days; rainfed fields yield about 600–700 kg of dry leaves and 300 kg of pods per hectare. In Sudan, PROTA reports two harvests a year, partly from wild plants. The active compounds are hydroxyanthracene glycosides. EMA requires senna leaf to contain at least 2.5% of them, calculated as sennoside B (PubChem CID 91440, C42H38O20), Alexandrian pods at least 3.4%, and Tinnevelly pods at least 2.2%. The main constituents are sennosides A and B, both rhein-dianthrone glycosides. Leaves contain more of the emodin and aloe-emodin types than pods, and free aglycones increase as the dried herb is stored, so the strength of a loose tea is hard to predict.

PROTA records that senna leaves and pods have been used as a purgative since ancient times and that the trade dates back at least to the ninth century AD. EMA’s assessment report says senna was introduced into European medicine by the Arabs in the ninth or tenth century, and Merriam-Webster traces the English word, first recorded in 1543, through New Latin to the Arabic sanā. Culbreth’s Manual of Materia Medica (1927) adds that “the Arabians used it in skin affections.” European herbalists took it up enthusiastically: EMA lists Lonicerus (1564), Bock (1565), and Matthiolus (1626) among writers describing its laxative use, notes that Matthiolus even claimed to cure syphilis with it and that Paracelsus used it as a purgative, and records senna leaves as an official laxative infusion in the Austrian Pharmacopoeia of 1812. In Africa and Asia senna had wider folk uses. PROTA reports that in Sudan a decoction of the pods was drunk against intestinal worms and an infusion of the leaves against flatulence and nosebleeds, while EMA cites Indian texts listing loss of appetite, liver disease, anemia, and leprosy, and the use of senna with henna as a black hair dye. WHO monographs list further folk uses, from expectorant to wound dressing, and note that they are not supported by experimental or clinical data.

In the drug trade, senna was sold by place of origin. Alexandrian or Khartoum senna came from the Nile valley, much of it gathered from wild plants, while Tinnevelly senna came from southern India. The ICAR bulletin says Cassia angustifolia was reportedly first introduced to the Tirunelveli district of Tamil Nadu in the mid-eighteenth century, hence the name Tinnevelly, the colonial spelling of the district, and it remains a cultivated crop there today. By about 1990 India exported 5,000 to 7,000 tonnes of leaves and pods a year, mainly to Germany, the United States, Japan, the Netherlands, France, Switzerland, and the United Kingdom, and in 2003 it exported 9,700 tonnes of leaves alone; Sudanese exports varied from 700 to 7,740 tonnes (PROTA). PROTA also notes that adulteration was once common and profitable, with up to 90% substitution by avaram (Senna auriculata) recorded, until importing countries tightened their controls.

In American medicine, Ellingwood’s Materia Medica (1919) called Alexandria senna “mild, kindly, certain and uniform in its action” and a constituent of most proprietary laxative syrups and elixirs, though Culbreth complained that its smell, taste, and tendency to nauseate lessened its popularity. Standardized senna tablets brought consistency: the current Senokot label lists a US marketing start date of 1956. Senna’s modern dominance owes something to regulators. Spiller and colleagues (Annals of Pharmacotherapy 2003) note that after the FDA had phenolphthalein removed from over-the-counter laxatives in 1999, most products replaced it with senna. In 2002 the FDA removed aloe and cascara sagrada from over-the-counter laxatives because no carcinogenicity data had been submitted, while noting that it had received the data it requested on senna and bisacodyl. Senna is still sold under the FDA’s over-the-counter monograph system. In the European Union, a 2021 regulation banned aloe-emodin, emodin, and aloe leaf extracts from foods and placed senna leaf and fruit preparations in foods under “Union scrutiny,” a watch list pending more safety data; licensed senna medicines are regulated separately. The EU General Court annulled parts of that regulation in November 2024, including the aloe-leaf ban; the Commission has appealed, so the final position is not yet settled.

Senna is a prodrug that the gut’s own bacteria switch on. EMA explains that sennosides are glycosides, molecules with sugars attached, linked in a way that human digestive enzymes cannot split, so they pass through the stomach and small intestine unabsorbed. In the large intestine, bacteria remove the sugars and convert them to the active metabolite, rhein anthrone. Very little enters the body: in animal experiments, under 10% of rhein anthrone placed directly in the cecum was absorbed, and in people taking senna pod powder equivalent to 20 mg of sennosides for seven days, blood rhein peaked at about 100 ng/ml without building up. About 90% of the sennosides leave in the stool as polymers, and 3–6% of metabolites appear in urine, which is why urine may turn yellow or red-brown. Because colonic bacteria activate it, the AGA–ACG guideline notes that antibiotics could in theory reduce the effect of stimulant laxatives such as senna.

EMA describes two actions. First, rhein anthrone stimulates the muscle of the large intestine, speeding colonic transit. Second, it changes fluid handling: it blocks the absorption of water, sodium, and chloride from the colon, loosens the tight junctions between lining cells, and stimulates secretion of water and electrolytes into the bowel, so stools stay softer and bulkier. The AGA–ACG guideline adds that rhein anthrone and rhein stimulate production of prostaglandin E2 and chloride secretion. The delay is built in by the time it takes to reach the colon and be activated: EMA gives 8–12 hours, US labels 6–12 hours, which is why senna is taken at bedtime. Trials of fiber with and without senna show the size of the push. In a study described by EMA, Marlett and colleagues (1987) found that stool frequency in chronically constipated adults rose from 2.3 to 3.6 a week on ispaghula husk alone but from 2.0 to 6.8 a week on psyllium plus senna, while cramping and other gut side effects were reported by 32% versus 14%.

Long-term effects are where debate lies. EMA reports that use of anthranoid laxatives for 4 to 13 months leads to melanosis coli, in which immune cells in the colon lining store a brown pigment, probably lipofuscin, from anthranoid breakdown; it was found in 62% of chronic users of these laxatives, compared with 12–31% of people with chronic constipation in general, and fades 6–12 months after stopping. LiverTox describes a “cathartic colon” with diarrhea, cramps, and weight loss after long-term abuse, but Morales and colleagues (Journal of Toxicology 2009), reviewing the literature, found no convincing evidence that chronic senna use damages the nerves or muscle of the bowel wall, and a two-year rat study found no changes in the colon’s nerve network in the few samples examined. Laboratory tests are mixed: sennosides, rhein, and sennidins were not mutagenic, while senna pod extracts and some related anthranoids were in test-tube systems, a finding not confirmed in animals for senna or aloe-emodin (EMA).

Occasional constipation is the core use, and the one regulators accept. EMA’s monographs, adopted in 2018 and published in 2020, classify senna leaf and pod as well-established medicines for short-term use in occasional constipation in adults and adolescents over 12. The single dose is a preparation supplying 10–30 mg of hydroxyanthracene derivatives, calculated as sennoside B, taken once at night, using the smallest dose that produces a comfortable soft-formed motion. Use is limited to one week, and EMA notes that two or three doses in that week are usually enough. EMA candidly acknowledges that there is no well-designed study of senna alone for occasional constipation: its judgment rests on pharmacology, expert reports, extensive clinical experience, and trials of senna-fiber combinations, which consistently showed an added laxative effect from the senna. US labels likewise state that senna relieves occasional constipation and generally produces a bowel movement in 6 to 12 hours.

Chronic constipation: the only placebo-controlled trial is by Morishita and colleagues (American Journal of Gastroenterology 2021), who randomized 90 Japanese adults with chronic idiopathic constipation (mean age 42, 93% women, symptoms for an average of 9.9 years) to senna 1.0 g, magnesium oxide 1.5 g, or placebo daily for 28 days. Overall improvement was reported by 69.2% on senna, 68.3% on magnesium oxide, and 11.7% on placebo, with more frequent spontaneous bowel movements and better quality of life on both active treatments and no severe adverse events. On this basis the 2023 AGA–ACG guideline (Chang and colleagues, Gastroenterology) suggests senna over no senna, a conditional recommendation with low certainty of evidence, and calls it an attractive first-line option given its effect, low cost, and availability. It flags the limits: one small trial, a dose higher than used in practice, 83% of senna takers reducing their dose during the trial, and no long-term efficacy or safety data. It suggests starting at 8.6–17.2 mg a day. Strong recommendations went to polyethylene glycol, sodium picosulfate (another stimulant laxative), linaclotide, plecanatide, and prucalopride.

Older adults: Passmore and colleagues (BMJ 1993) ran a double-blind crossover trial in 77 long-stay patients with an average age of about 83, in hospitals and nursing homes in Northern Ireland and England. Each took a senna-fiber combination (10 ml daily) and lactulose (15 ml twice daily) for 14 days each. Bowel movements averaged 0.8 a day on senna-fiber and 0.6 on lactulose, stool consistency and ease of passage were better, adverse effects did not differ, and the cost per stool was 10.3p against 39.7p. A smaller crossover trial by Kinnunen and colleagues (1993), described by EMA, found about 4.5 bowel movements a week on a senna-fiber combination versus about 2 on lactulose in 30 elderly patients. Because these products combined senna with plantago fiber, they cannot show what senna does alone, and MedlinePlus advises that older adults should not usually take senna long term, because other medicines are safer for them.

Opioids, surgery, and childbirth: Agra and colleagues (Journal of Pain and Symptom Management 1998) randomized 91 terminally ill cancer patients starting opioids to senna or lactulose for 27 days and found no difference in days with a bowel movement or in three-day stretches without one, and recommended senna because it cost less. A Cochrane review by Candy and colleagues (2015) found only five small trials of laxatives in palliative care, which suggested similar effectiveness among them, but the evidence was too limited to rank them. EMA’s assessment notes that senna is widely used and recommended over lactulose in the UK for people taking opioids, mainly on cost. After pelvic reconstructive surgery, Patel and colleagues (American Journal of Obstetrics and Gynecology 2010) found that senna 8.6 mg with docusate shortened the time to a first bowel movement from 4.05 to 3.0 days compared with placebo, and fewer patients needed magnesium citrate (7.0% versus 43.6%). An older South African trial of Senokot after childbirth (Shelton, 1980) reported success in 93–96% of women, against 51–59% with placebo.

Bowel preparation and other uses: EMA accepts senna pod for bowel cleansing before procedures such as colonoscopy, as a single dose of 150 mg of hydroxyanthracene derivatives in the early afternoon of the day before, with plenty of clear fluids; it notes that standard polyethylene glycol or sodium phosphate preparations were slightly more effective, but senna caused less vomiting and more cramps. Tian and colleagues (Gastroenterology Nursing 2022) pooled 11 trials with 3,343 patients and found no significant difference in bowel cleanliness between senna regimens and others, with better tolerance and adherence. Senna is not a treatment for irritable bowel syndrome: a 284-patient trial of tablets combining charcoal, senna, and rhubarb could not show a benefit from the senna, and EMA warns that senna can cause pain and cramping particularly in people with an irritable colon. It is useless for weight loss: Bo-Linn and colleagues (Annals of Internal Medicine 1983) found that even purging that produced 4 to 6 liters of diarrhea cut calorie absorption by only about 12%.

Most senna sold in the United States is standardized tablets. A typical Senokot label lists 8.6 mg of sennosides per tablet and advises taking it preferably at bedtime: adults and children 12 and over start with 2 tablets once a day, up to a maximum of 4 tablets twice a day; children 6 to under 12 take 1 tablet once a day, up to 2 twice a day; children 2 to under 6 take half a tablet, up to 1 twice a day; and for children under 2 the label says to ask a doctor. MedlinePlus notes that senna also comes as liquids, powders, granules, chewable pieces, and combinations with the stool softener docusate, such as Senokot-S and Peri-Colace. EMA is more cautious than the US label, contraindicating senna under 12, recommending a single nightly dose of 10–30 mg of hydroxyanthracene derivatives, and requiring products to allow lower doses so that people can find the smallest one that works.

EMA also accepts standardized senna leaf and pod as herbal teas, but loose teas are harder to dose. The AGA–ACG guideline notes that senna is present in many laxative teas, where the dose may be difficult to calculate. Senna leaf must contain at least 2.5% hydroxyanthracene glycosides, so a single gram of dried leaf supplies at least about 25 mg, already near the top of EMA’s single dose, and actual content varies with the crop, the plant part, and storage. LiverTox notes that senna is included in several herbal teas sold for purging and weight loss. The most serious case of liver failure linked to senna involved a woman who had drunk a liter a day of tea brewed from a 70 g bag of dried senna pods for more than three years (Vanderperren, Annals of Pharmacotherapy 2005). Products standardized to a stated sennoside content are the safest way to know what you are taking.

Practical use: take senna at bedtime with water, expect a bowel movement the next morning, and keep drinking fluids. Start with the lowest dose and increase only if needed. The NHS advises seeing a doctor if constipation has not improved after three days of senna and not taking it for more than seven days unless a doctor or pharmacist advises it; US labels say to stop and ask a doctor if there is rectal bleeding or no bowel movement after using a laxative, because these may signal a serious condition. EMA recommends senna only when changes to diet or a bulk-forming fiber have not worked. MedlinePlus advises taking some senna products at least two hours apart from other oral medicines. People who have used stimulant laxatives for a long time and cannot go without them should, as NIDDK suggests, ask their doctor how to stop slowly; over time the colon should start moving stool normally again. EMA advises changing pads more often in incontinent adults to avoid skin damage from prolonged contact with stool.