Can't defecate. Stool becomes hard and dry; it's difficult for the colon to move it along, and difficult (and painful) for the hard stool to pass through the anal canal. Lack of water in diet can contribute; chronic use of opiate pain medscan too. Rarely people can have motility disorders of the intestines that contribute to fecal impaction. In extreme cases hard stool can wear through the intestinal wall.