Narrow stools that happen now and then probably are harmless. But in some cases, narrow stools — especially if pencil thin — may be a sign that the colon is narrowing or has a blockage. And that could be due to colon cancer.
Irritable bowel syndrome (IBS) also may cause changes in the size of your stools. IBS can cause stools to be smaller, larger or narrower than usual. It also causes changes in the consistency of stools.
Check with a healthcare professional if you notice any changes in the appearance of your stool that last longer than 1 to 2 weeks. Get medical help right away if your bowel changes happen with rectal bleeding or severe belly pain.