Colon cancer and irritable bowel syndrome can cause narrowing of the stool, notes Mayo Clinic. Infrequent occurrences of narrow stool are usually not serious, however.
Pencil-thin or otherwise narrow stools may result from an obstruction or narrowing of the colon due to cancer, reports Mayo Clinic. In addition, irritable bowel syndrome often causes a change in stool size and consistency. While most instances of narrow stools are benign and no cause for worry, narrower-than-normal stools that occur for longer than a couple of weeks require evaluation by a medical doctor. Narrow stool accompanied by severe abdominal pain or rectal bleeding require immediate evaluation by a medical professional.