Frangula (commonly known as Frangula alnus or buckthorn bark) has a longstanding traditional use as a herbal remedy for constipation and to support bowel movements, hence its association with the rectum and lower digestive tract. The bark contains anthraquinone glycosides, which are naturally occurring compounds known to stimulate bowel movements by increasing peristalsis in the colon. This laxative action has been recognized in European herbal medicine for centuries, with historical references to its use dating back to the Middle Ages. In fact, Frangula bark was included in several traditional pharmacopeias as a treatment for constipation and related digestive complaints.

While there is traditional backing for Frangula's use, scientific validation is limited but not absent. Some pharmacological studies have confirmed the presence of laxative anthraquinones and described their mechanism of action, which involves irritating the colon lining to promote water and electrolyte secretion and stimulate motility. However, robust clinical trials in humans are sparse, and most evidence comes from animal studies or extrapolation from related plants, such as Rhamnus purshiana (cascara sagrada). Because of this, regulatory authorities, such as the European Medicines Agency (EMA), recognize its traditional use for short-term constipation relief, but caution against prolonged or excessive use due to potential side effects like electrolyte imbalance or dependency.

In summary, Frangula is primarily justified by traditional use in supporting rectal elimination, with moderate evidence (rated 3/5) owing to historical use and some pharmacological support, but limited modern clinical validation.