Senna is a shrub found in India, Pakistan, South China and elsewhere. Its name is derived from the Arabic word "sena," and it has been used in ayurvedic and unani medicines since the ninth century. The shrub grows to a height of approximately two feet, with green stems and pods and yellow, scoop-shaped leaves. The alternate leaves are even-pinnate, with four or five pairs of lanceolate or obovate leaflets that are brittle and greyish green in colour. The small yellow flowers are characterised by five spreading clawed petals. The fruit is in an oblong pod about 5cm long. The leaves and pods, or fruit, are used medicinally.

History

People in northern Africa and southwestern Asia have used senna as a laxative for centuries. Senna was traditionally considered a "cleansing" herb because of its cathartic properties. More modern studies have found that senna is useful in treating constipation, whether it is caused by pharmaceuticals or natural means. In addition, the leaves were sometimes made into a paste and applied to various skin diseases. Ringworm and acne were both treated in this way.

Function

The active substances in senna leaves are called sennosides. These molecules are converted by bacteria in the colon into another substance, rhein-anthrone, which has two beneficial effects, colon activity stimulation(which speeds bowel movements and improves digestion) and increased fluid secretion. The laxative effect of sennosides and their active metabolite, rhein anthrone, is due to inhibition of water and electrolyte absorption from the large intestine, which increases the volume and pressure of the intestinal contents. This will stimulate the colon motility resulting in propulsive contractions. In addition, stimulation of active chloride secretion increases water and electrolyte content of the intestine. These changes in active electrolyte transport are depend on calcium in the serosal surface. The laxative action of Sennosides are partially via stimulation of colonic fluid and electrolyte secretion, and this secretion is mediated by stimulation of endogenous prostaglandin formation ². Sennosides may be prepared as an enema or suppository or combined with a Stool Softener or Bulk Forming Fiber Laxative to form a Combination Laxative.³

Chemistry

Senna leaf contains: Approximately 3% dianthrone glycosides (sennosides A, A1, B, C, D, E, F & G ); and small amounts of anthraquinones including aloe-emodin and rhein 8-glucoside; approximately 10% mucilage; tannins; and flavonoids. ⁴ The extract is standardized to 8% sennosides.This product is mainly composed of Sennosides. The structural formulas are followed:

Sennoside A:  C₄₂H₃₈O₂₀                                                                   Sennoside C: C₄₂H₄₀O₁₉

Dosage:

100-150 mg at bedtime as purgative. ⁵

Safety:  

Sennosides are strong purgative that should be taken with care and in proper dosage, especially for pregnant, menstruating, or postpartum women. It cannot be used where there is inflammation in the G.I. Tract because of irritation. The irritant effect upon the intestinal membrane may cause griping, pain or nausea, along with liquid stools or diarrhea for overdose. 

References:

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8. Mengs U. Reproductive toxicological investigations with sennosides. Arzneimittelforschung 1986;36:1355–8.

9. Faber P, Strenge-Hesse A. Relevance of rhein excretion into breast milk. Pharmacol 1988;36(suppl 1):212–20.