Narrow stools that occur on an infrequent basis are probably not an indication of a serious underlying problem. However in some cases, narrow stools — especially if pencil thin — may be a sign of narrowing or obstruction of the colon due to colon cancer. In addition, irritable bowel syndrome may cause irregular stools that alternate between normal and either constipation or diarrhea. Such stools may also be narrower than usual.
Check with your doctor if you notice any changes in your bowel habits — such as narrower than normal stools — that last longer than two weeks. Consult your doctor immediately if your bowel changes are accompanied by rectal bleeding or severe abdominal pain.