Senna is a stimulant laxative used in the treatment of constipation, according to WebMD. Senna may also be used to clean out the intestines before a patient undergoes bowel examination, or surgery.
Senna works by keeping water within the intestines, rendering feces more fluid and passable. The medication is taken by mouth, either as a tablet or in liquid form, as stated by WebMD. It may take six to 12 hours before the medication causes a bowel movement. Common side-effects associated with senna include abdominal pain, stomach cramps, nausea, diarrhea and weakness. The medication may also turn urine a reddish-brown color.