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.

Does thin stools always mean cancer?

Sometimes size is a concern

In certain types of colon cancer, the bowel gets narrow, and so do your bowel movements. Thin stools do not automatically mean cancer.

How narrow should stools be?

Normal stool is about one to two inches in diameter. Stringy poop is narrow and, in some cases, almost flat, giving it a stringy appearance. It may be solid or loose.