The correct dose of senna, when treating constipation in adults and children age 12 and older, is 17.2 milligrams daily, according to WebMD. The recommended daily maximum is 34.4 milligrams.
In children, doctors recommend 8.5-milligram doses of senna for relief of constipation, while 28 milligrams of senna given in two doses is effective for treating constipation following pregnancy, reports WebMD. The U.S. Food and Drug Administration approved senna, which is an herb, for use as a nonprescription laxative. Other uses for senna include clearing the bowel prior to diagnostic testing, treating hemorrhoids and alleviating symptoms of irritable bowel syndrome. Senna contains sennosides, which are chemicals that irritate the bowel’s lining, causing bowel movements.