Senna contains hydroxyanthracene glycosides known as sennosides. These glycosides stimulate colon activity and thus have a laxative effect. Also, these glycosides increase fluid secretion by the colon, with the effect of softening the stool and increasing its bulk.¹ Double-blind trials have confirmed the benefit of senna in treating constipation.^{2, 3} Constipation induced by drugs such as the anti-diarrhea medicine loperamide (Imodium®) has also been shown to be improved by senna in a clinical trial.⁴

A double-blind trial showed that senna was more effective as a preparatory agent for bowel surgery than the commonly used polyethylene glycol (PEG).⁵ Patients scheduled to undergo bowel surgery received either 120 mg of senna in a glass of water or 118 mg of PEG in about 2–3 quarts of water the night before surgery. Surgeons rated the efficacy of senna at clearing the bowels at 70%, compared to 58% efficacy for PEG. Supplementation with senna for this purpose should always be supervised by the surgeon.