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What Does Your Stool Look Like if You Have Colon Cancer?

Ask a Doctor

I recently noticed changes in my stool color – it’s been much darker for the last week or so. Colon cancer runs in my family, so I’m afraid I might have it. What does your poop look like if you have colorectal cancer?

Doctor’s Response

New onset of bright red blood in the stool always deserves an evaluation. Blood in the stool may be less evident, and is sometimes invisible, or causes a black or tarry stool.

People commonly attribute all rectal bleeding to hemorrhoids, thus preventing early diagnosis owing to lack of concern over "bleeding hemorrhoids."

Blood usually, but not always, can be detected through a fecal occult (hidden) blood test, in which samples of stool are submitted to a lab for detection of blood

Symptoms of colon cancer may not be present or be minimal and overlooked until it becomes more severe. Cancer screening tests for colon cancer thus are important in individuals 50 and older. Cancer of the colon and rectum can exhibit itself in several ways. If you have any of these symptoms, seek immediate medical help..

  • Rectal bleeding may be hidden and chronic and may only show up as an iron deficiency anemia.
  • It may be associated with fatigue and pale skin due to the anemia.
  • Changes in bowel movement frequency
  • It usually, but not always, can be detected through a fecal occult (hidden) blood test, in which samples of stool are submitted to a lab for detection of blood.
  • If the tumor gets large enough, it may completely or partially block your colon. You may notice the following symptoms of bowel obstruction:
    • Abdominal distension: Your belly sticks out more than it did before without weight gain.
    • Abdominal pain: This is rare in colon cancer. One cause is tearing (perforation) of the bowel. Leaking of bowel contents into the pelvis can cause inflammation (peritonitis) and infection.
    • Unexplained, persistent nausea or vomiting
    • Unexplained weight loss
    • Change in frequency or character of stool (bowel movements)
    • Small-caliber (narrow) or ribbon-like stools
    • Constipation
    • Sensation of incomplete evacuation after a bowel movement
    • Rectal pain: Pain rarely occurs with colon cancer and usually indicates a bulky tumor in the rectum that may invade surrounding tissue after moving through the colon's submucosa.

Studies suggest that the average duration of symptoms (from onset to diagnosis) is 14 weeks.

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Reviewed on 4/26/2018
American Cancer Society. "Colorectal Cancer Statistics, 2017." 2017. <http://pressroom.cancer.org/CRCstats2017>.

Cancer.Net. "Colorectal Cancer: Treatment Options." August 2017. <http://www.cancer.net/cancer-types/colorectal-cancer/treatment-options>.

"Clinical presentation, diagnosis, and staging of colorectal cancer"

"Overview of the management of primary colon cancer"

"Colorectal cancer: Epidemiology, risk factors, and protective factors"