The short answer
Blood in the stool is most often caused by common, harmless problems such as hemorrhoids or small tears — not cancer. Still, because it can occasionally be a sign of something that needs attention, blood in the stool is always worth having checked by a doctor.
Blood in the stool is most often caused by common, non-cancer problems like hemorrhoids.
It is still worth having checked, because the cause is not always obvious.
Bright red blood and dark, tarry stool can point to different sources.
Blood with weight loss, a change in bowel habits, or belly pain deserves prompt review.
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The full explanation.
The simple version
Seeing blood in the toilet or on the paper is alarming. But blood in the stool is most often caused by common, harmless problems — not cancer. The cause is not always obvious from the outside, so it is worth having a doctor check.
What usually causes it
The most frequent causes are ordinary and treatable:
- Hemorrhoids. These are swollen veins near the rectum, and they are very common.
- Anal fissures. These are small tears in the lining near the anus.
- Irritation from straining or hard stools.
Some foods, supplements, and medicines can also change the color of stool in ways that look like blood. None of these is cancer.
What the color can suggest
Color offers a clue about where blood may be coming from. Bright red blood often comes from lower down, near the rectum or anus. That is the typical pattern with hemorrhoids. Dark, tarry stool can come from higher up in the digestive tract. Color helps guide a doctor, but on its own it does not diagnose the cause.
Patterns more worth checking promptly
Have blood in the stool looked at sooner rather than later if it:
- Is heavy, or keeps happening.
- Comes with a lasting change in bowel habits.
- Comes with unexplained weight loss or belly pain.
- Leaves you feeling weak, dizzy, or short of breath.
These patterns have many non-cancer causes too, but they deserve prompt attention.
When to get help sooner
- Call 911 or go to an emergency department if you are passing a large amount of blood, or you feel faint, dizzy, cold, clammy, or your heart is racing.
- Call 911 or go to an emergency department if you vomit blood, or vomit something that looks like coffee grounds.
- Call your care team the same day if your stool is black and tarry, or you have bad belly pain along with the bleeding.
- Call your care team the same day if you also have a fever, or you feel unusually short of breath or weak.
- Call your care team within a day or two if the bleeding keeps happening, comes with a lasting change in your bowel habits, or comes with weight loss you cannot explain.
When to see a doctor
It is reasonable to have any blood in the stool checked, even once. A doctor can examine you and ask about the pattern. They can then decide whether tests such as a stool test or a colonoscopy are needed. Most of the time the cause turns out to be minor. It is still worth an exam even if you are sure hemorrhoids are the cause, because that is not something you can confirm on your own.
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Common questions
Does blood in the stool mean I have cancer?
Usually not. The most common causes are harmless, such as hemorrhoids or a small tear in the anal lining. But because blood in the stool can occasionally signal something that needs treatment, it is always worth having a doctor check it rather than assuming the cause.
What usually causes blood in the stool?
Common causes include hemorrhoids, small tears (anal fissures), and irritation. Diet and certain foods or medicines can also change stool color in ways that look like blood. A doctor can tell what is actually going on and whether testing is needed.
What does the color of the blood mean?
Bright red blood often comes from lower down, near the rectum or anus, such as from hemorrhoids. Dark, tarry stool can come from higher up in the digestive tract. Color is a clue, not a diagnosis — a doctor uses it along with other information.
When should I see a doctor about blood in my stool?
It is reasonable to have any blood in the stool checked. See a doctor promptly if it is heavy, keeps happening, or comes with weight loss, a lasting change in bowel habits, belly pain, or feeling weak or dizzy.
Questions to ask your doctor
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Sources last checked: 2026-07-14 what this meansLast updated: 2026-08-11Next planned review: 2027-07-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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