Dulcolax Suppositories contains bisacodyl which is used for the treatment of constipation. Bisacodyl works by stimulating the nerve endings in the walls of the large bowel (colon) and rectum. This makes the muscles in the bowel wall contract more often and with increased force which moves the stools through the colon to the rectum so that the bowel can be emptied. Dulcolax suppositories melt in the rectum releasing the bisacodyl to produce its effect on the wall of the colon and rectum. The suppositories start to work in about 15 to 30 minutes so they are useful if you want fast relief from constipation.