Senna is an herb. The leaves and the fruit of the plant are used to make medicine. It is used to treat constipation and also to clear the bowel before diagnostic tests such as colonoscopy and weight loss. Senna takes about 8 hours to work. It’s normal to take it at bedtime so it works overnight. Drink plenty of fluids (6 to 8 glasses a day) while you are taking Senna or your constipation may get worse.