What are the symptoms of colon cancer?

Short answer: The most common early symptoms of colon cancer are blood in the stool, a persistent change in bowel habits (new constipation or diarrhea lasting more than a few weeks), unexplained weight loss, ongoing abdominal pain or cramping, and fatigue from unexplained iron-deficiency anemia. Any of these in someone over 45 — or with a family history — is a same-week doctor visit. Details and screening guidance below.

The short answer: the most common early symptoms of colon cancer are a change in bowel habits lasting more than a few weeks (new constipation, diarrhea, or narrower stools), blood in the stool or rectal bleeding, ongoing abdominal cramps or bloating, unexplained weight loss, and fatigue from low iron. Many people have no symptoms at all in the early stages, which is why screening matters more than waiting for a sign.

Colon cancer (also called colorectal cancer when it includes the rectum) is one of the most common cancers in the U.S. — and one of the most treatable when caught early. The tricky part is that early-stage colon cancer is often silent, and the symptoms that do appear look like a dozen ordinary problems: hemorrhoids, IBS, a bad week of eating, stress. This guide walks through what the symptoms actually are, which ones are worth calling a doctor about today, and what happens next if you do.

The most common symptoms — in plain language

Not everyone has all of these, and having one doesn't mean you have cancer. But these are the signs oncologists and gastroenterologists ask about first.

• A change in your bowel habits that lasts more than 3 to 4 weeks. New constipation, new diarrhea, or a pattern that's suddenly different from your normal — and doesn't go back on its own.
• Blood in the stool or on the toilet paper. Bright red blood is often from hemorrhoids, but it can also be from the rectum or lower colon. Dark, tarry stools can mean bleeding higher up. Either is worth getting checked.
• Stools that look narrower or "pencil-thin" for more than a couple of weeks.
• Ongoing cramping, gas, or bloating in the lower abdomen — especially if it's new for you.
• A feeling that your bowel doesn't fully empty after you go.
• Unexplained weight loss — losing weight without trying, especially more than 10 pounds.
• Fatigue and weakness from anemia (low iron), often because of slow, unnoticed bleeding.
• Low blood counts on a routine blood test — sometimes the first clue, before any symptoms show up.

Takeaway: it's rarely one dramatic symptom. It's usually a small change that stays.

Which symptoms need a doctor call today

Some symptoms deserve a same-week appointment, not a "let's see if it goes away."

• Visible blood in the stool — even once, even if you have hemorrhoids.
• Black or tarry stools.
• A change in bowel habits that has lasted more than 3 weeks.
• Weight loss you can't explain.
• New, persistent abdominal pain.
• Iron-deficiency anemia flagged on a blood test, especially in anyone over 40.

If any of these apply, the next step is usually a call to your primary care doctor, who will either order labs, refer you to a gastroenterologist, or recommend a colonoscopy.

Takeaway: blood, unexplained weight loss, and low iron are the three findings doctors take most seriously in adults over 40.

Symptoms by stage — why early colon cancer is often silent

Colon cancer symptoms usually get louder as the cancer grows, which is why early screening matters so much more than waiting for a sign.

• Stage 0 to I: often no symptoms at all. Found on a screening colonoscopy or an incidental blood test.
• Stage II: occasional bleeding, mild changes in bowel habits, sometimes cramping.
• Stage III: more consistent bleeding, persistent bowel changes, fatigue from anemia, sometimes weight loss.
• Stage IV (spread beyond the colon): symptoms depend on where it has spread — liver (jaundice, right-side pain), lungs (cough, shortness of breath), or belly (fluid buildup, called ascites).

Takeaway: if you wait for symptoms, you're usually catching it at stage II or later. A screening colonoscopy catches most cases at stage 0 or I, when it's almost always curable.

Who is at higher risk

Anyone can develop colon cancer, but the risk goes up with:

• Age 45 and older — the U.S. Preventive Services Task Force now recommends screening starting at 45, down from 50.
• A family history of colon cancer or polyps, especially in a parent or sibling.
• Personal history of inflammatory bowel disease (Crohn's or ulcerative colitis).
• Certain inherited conditions — Lynch syndrome, FAP.
• A diet high in processed and red meats, low in fiber; heavy alcohol use; smoking; obesity; type 2 diabetes.
• Black Americans have both higher incidence and higher mortality, and often benefit from starting screening earlier.

Takeaway: if you have a first-degree relative who had colon cancer, start screening 10 years before the age they were diagnosed — or at 40, whichever comes first.

What screening actually looks like

Most people picture colonoscopy and stop there, but there are several options.

• Colonoscopy — the gold standard. Every 10 years for average risk. Finds and removes polyps in the same visit.
• Stool-based tests (FIT, Cologuard) — done at home, every 1 to 3 years depending on the test. A positive result means you still need a colonoscopy.
• CT colonography ("virtual colonoscopy") — every 5 years. Doesn't remove polyps; a positive finding still means a colonoscopy.
• Flexible sigmoidoscopy — less common now, every 5 years.

The best screening test is the one you'll actually do. If colonoscopy feels like a barrier, a FIT or Cologuard test is far better than nothing.

Takeaway: screening isn't optional over 45. Pick the test you'll actually complete.

What to do if you have symptoms

If you recognize yourself in this list, the next steps are usually straightforward.

1. Write down what you've noticed — what changed, when it started, how often, any blood, any weight change. Doctors ask all of this on the first visit.
2. Call your primary care doctor for an appointment this week, not next month. Say "I'm having symptoms I'd like to rule out colon cancer" — that gets you triaged appropriately.
3. Ask for a stool test or a gastroenterology referral if the doctor isn't sure. Both are reasonable next steps.
4. Bring someone with you if you're anxious. Two people hear more than one.
5. Don't wait for the symptom to go away on its own. Most benign causes (hemorrhoids, IBS) don't need to be ruled in — the dangerous ones need to be ruled out.

If you're diagnosed, our guide to what to do in the first 72 hours after a cancer diagnosis walks through the exact order of steps that matter most. And if your first appointment left you with a report full of words you don't understand, how to read a pathology report you don't understand is a plain-language walkthrough.

Takeaway: most people who go in with these symptoms don't have cancer. But the ones who do get a five-year survival rate near 90 percent when it's caught early — and that number drops sharply once it spreads.

Related reading

• Just diagnosed with cancer: what to do first — the first 72 hours, in order.
• How to read a pathology report you don't understand — a plain-language walkthrough.
• Questions to ask after any new diagnosis — the full framework for the first real appointment.

---

This post is for general informational purposes and isn't a substitute for guidance from your care team. If you have symptoms that concern you, call your doctor.