There have been a limited number of clinical trials on cascara sagrada that have explored its value for cleansing the colon before bowel examinations and similar diagnostic tests. Its value is clear for easing constipation--when it's taken properly and at a safe dosage. In fact, cascara is such a mild laxative that it can safely be used by the elderly, and for the mild constipation that can occur following anal or rectal surgery.
Specifically, cascara sagrada may help to:

Control occasional constipation. Early plant chemists identified the active laxative constituents in cascara sagrada bark: anthraquinone derivatives. These compounds stimulate peristalsis, the vigorous wavelike contractions of the large intestine that keep food moving through the digestive system. When cascara speeds the process up, the body produces a softer, quicker bowel movement because the intestine has had less chance to absorb the liquid from the stool. Several studies have shown that cascara sagrada is effective in easing chronic constipation in elderly people.
Treat hemorrhoids and anal fissures. Combined with conventional measures to soften the stool (plenty of dietary fiber, water, and exercise), occasional use of cascara sagrada preparations makes sense for preventing the pressure and pain associated with hemorrhoids and anal fissures (cracks in the skin near the anus). In fact, by speeding up bowel movements, hemorrhoids are also less likely to develop.