Narrow stools that occur infrequently probably are harmless. 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.
Irritable bowel syndrome (IBS) is another condition that may cause changes in the size of your stools, so that they're smaller, larger or narrower than usual. IBS also causes changes in the consistency of stools.
Check with your doctor if you notice any changes in your bowel habits — such as narrower than normal stools — that last longer than one to two weeks. Consult your doctor immediately if your bowel changes are accompanied by rectal bleeding or severe abdominal pain.