Extracts from "Laxatives and the Special Role of Senna"
By E.W. Godding, Pharmacology 36 (1988)

The most widely used anthranoid drug is senna which contains mainly sennasides. Sennosides are glycosides of rhein dianthrone with a high molecular weight. They are produrgs which are not absorbed in the upper gut and which are split by bacterial enzymes in the colon. Their active metabolite is rhein monoanthrone.

Sennosides belong to the secretagogue drugs as they induce net secretion of fluid. They do, however, not change mucosal permiablity and do not damage the intestinal mucosa. Their mechanism of action is not fully understood but may include mediators like the prostaglandins and serotonin.

Apart from their secretagogue effect, there seems to be general agreement that sennosides specifically influence colonic motility and enhance colonic transit. This is important as constipation is a motility problem and a laxative should preferentially normalise the disturbed motility.

The time of action of senna is usually stated as 8-10 hours so that taken at night it acts the next morning. It is, however, a simple dose-response relationship since, as already stated, its action augments, without disrupting, the response to the physiological stimuli of eating and physical activity.

In nursing mothers, the active principles may appear in the milk but insufficient amounts to induce diarrhoea in the suckling infant.

The naturally occurring senna glycosides have never been synthesised. Numerous studies have demonstrated that their action is much more than that of a laxative in the usual sense.

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