The short answer
Changes in bowel habits — such as diarrhea, constipation, or narrower stools — are usually caused by diet, stress, medicines, or infections, not cancer. A change that lasts more than a few weeks, or comes with blood in the stool or weight loss, is worth getting checked for colorectal cancer.
Most changes in bowel habits come from diet, stress, medicines, or infection.
A change lasting more than a few weeks is worth checking.
Blood in the stool or weight loss with it makes checking more important.
Early-stage colorectal cancer is often treated with surgery alone, so finding it early matters.
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This page is for anyone who has noticed a change in their bowel habits. It explains which changes matter, what else can cause them, and when to call your care team.
The short answer
Usually not, but a lasting change is worth checking. Diet, stress, and stomach bugs explain most short-term bowel changes. A change that lasts more than a couple of weeks can be a warning sign of colorectal cancer, a cancer of the colon or rectum.
What kind of change matters
Doctors pay attention to a change from your normal pattern that sticks around. This includes new constipation, new diarrhea, or stools that look narrower than usual. A feeling that your bowel does not fully empty after a movement is another sign worth mentioning. One or two days of an upset stomach after a bad meal is not the pattern doctors mean. Weeks of a new, different bowel pattern is.
Why colorectal cancer changes bowel habits
NCI lists these as possible signs of colon cancer: a change in bowel habits, diarrhea, constipation, and stools that are narrower or shaped differently than usual. Other warning signs include weight loss you cannot explain, belly discomfort, fatigue, and vomiting. Blood in the stool, bright red or very dark, is another warning sign NCI lists.
Far more common causes of bowel changes
Diet changes, new medicines, stress, infections, and irritable bowel syndrome can all change bowel habits. These are not cancer. Your doctor can help sort out which is the cause. Hemorrhoids are a common cause of bleeding. Blood in the stool still needs to be checked.
What a doctor checks next
Your doctor asks exactly what has changed and for how long. They ask whether you have seen blood, and whether you have lost weight without trying. A stool test that checks for hidden blood, such as the fecal immunochemical test (FIT), may be used. A colonoscopy lets the doctor look at the whole colon with a thin, tube-like instrument with a light and a lens. Tissue samples can be taken during the same procedure.
What a normal test does not rule out
Stool tests look for hidden blood, but a normal result does not explain a lasting change. If your bowel habits stay changed after a normal result, go back to your doctor and ask whether a colonoscopy is needed.
Screening matters even without symptoms
The CDC says most people should begin colorectal cancer screening at age 45, even with no symptoms. The USPSTF recommends screening for ages 45 to 75. The CDC says screening aims to find abnormal growths before they become dangerous. During a colonoscopy, the doctor can find and remove most polyps and some cancers. If you are 45 or older and have never been screened, mention that at your visit, whether or not your bowel habits have changed.
When to get help sooner
- Call your care team right away if you see blood in the toilet bowl or on the paper. Call too if cramping belly pain has come on with the change.
- Call your care team within a day or two if the new pattern has lasted beyond a couple of weeks. That means diarrhea, constipation, thinner stools, or a bowel that never feels empty. Call too if you have dropped weight without meaning to.
What to ask your doctor
- What has changed [describe], and how long has it lasted?
- Should I have a stool test or go straight to a colonoscopy?
- Am I due for colorectal cancer screening based on my age?
- What symptoms should bring me back sooner?
Sources
Common questions
Does a change in bowel habits mean cancer?
Usually not. Most changes come from diet, stress, medicines, or infection. A change lasting more than a few weeks is worth checking, though.
What change is more worth checking?
Ongoing diarrhea or constipation, thinner stools, or incomplete emptying — especially with blood in the stool, weight loss, or belly pain.
How does screening help?
Colorectal cancer screening, such as a colonoscopy or stool test, can find cancer or pre-cancer before symptoms appear. The CDC says a colonoscopy can find and remove most polyps and some cancers.
What non-cancer conditions cause bowel changes?
Diet changes, stress, medicines, infections, irritable bowel syndrome, and hemorrhoids are common causes.
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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, and this 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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