Senna is an FDA-approved over-the-counter (OTC) laxative. A prescription is not required to purchase a senna. It is used to treat constipation and also to clear the bowel before diagnostic tests such as colonoscopy. Senna is also used for irritable bowel syndrome (IBS), anal or rectal surgery, tears in the lining of the anus (anal fissures), hemorrhoids, and weight loss.