Is Blood in Your Stool a Sign of Cancer?

Usually not, but it should never be ignored. Hemorrhoids and other minor causes explain most blood in the stool. Blood can also be a sign of colorectal cancer, so it always deserves a doctor's evaluation rather than a guess.

Blood in the stool can look different depending on where it comes from. Bright red blood often points to bleeding lower down, near the rectum or anus. Very dark or black stool can mean bleeding higher up in the digestive tract. Sometimes there is no visible blood at all. Only a small, hidden amount shows up, and a stool test can detect it. This is one reason routine colorectal screening exists. It can catch bleeding you would never notice on your own.

A tumor in the colon or rectum has a fragile surface. Stool passing over it can make it bleed. This bleeding is often slow and comes and goes, not dramatic. That is part of why it can go unnoticed for a while. Over time, slow blood loss can also cause anemia, a low red blood cell count. This can show up as fatigue or pale skin, even before anyone notices blood directly.

Hemorrhoids, swollen veins in the rectum or anus, are an extremely common cause of visible blood, especially bright red blood. Anal fissures, small tears from constipation or straining, cause similar bleeding. Infections and inflammatory bowel disease can also cause blood in the stool. Certain foods, like beets, and some medicines can change stool color in ways that look like blood but are not.

Your doctor asks about the color and amount of blood. They ask whether you have pain, weight loss, or a change in bowel habits. A physical exam, including a rectal exam, can find hemorrhoids or fissures right away in many cases. If the cause is not obvious, or if you are 45 or older, a colonoscopy is usually the next step. A thin, flexible tube with a camera examines the entire colon. Doctors can remove any abnormal tissue for a biopsy during the same procedure. NCI puts the usual wait for a pathology report at up to 10 days after the procedure.

Finding hemorrhoids explains bleeding for many people, but it does not automatically rule out a second cause higher up. If bleeding continues after hemorrhoids are treated, or if you are due for colorectal screening, follow up rather than assuming the hemorrhoids explain everything.

The CDC recommends regular colorectal cancer screening starting at age 45 for most adults, even without symptoms. A yearly stool test, called a fecal immunochemical test, can pick up hidden blood you would never notice. A colonoscopy, usually done every ten years for average risk, both looks for bleeding and removes polyps — small growths that can turn into cancer — before they cause any bleeding at all.

• Call 911 or go to an emergency department if you pass a large amount of blood, your stools look black and tarry, or bleeding comes with dizziness, a racing heart, or severe pain in the belly.
• Call your care team the same day if bleeding keeps repeating over several days, or you feel unusually weak or breathless with it.
• Call your care team within a day or two if you see blood once and nothing obvious explains it, even when you feel entirely well.

• What could be causing this bleeding?
• Do I need a colonoscopy, or can we start with an exam?
• Am I due for colorectal cancer screening based on my age?
• What amount or pattern of bleeding should send me to urgent care?

• National Cancer Institute — Colon Cancer Treatment (PDQ)
• CDC — Screening for Colorectal Cancer
• National Cancer Institute — Pathology Reports