Poop and Colon Cancer: What Your BMs Reveal About Your Risk

by Erin L. Boyle Health Writer

Poo-poo, feces, bowel movements, stool—while we have A LOT of words to describe it, going to the bathroom is not an easy topic to discuss. But it’s a very necessary one to learn more about because, as it turns out, your poop is a kind of barometer of internal health. It can tell you if you’re sick, and even give you signs of something as serious as colon cancer…if you know what to look for. Join us for a guide to your poop—and how to know if you should seek testing for colon cancer.

toilet check

Check Before You Flush

As odd as it might seem, getting comfy with your bowel movements is important. So take a look in the bowl before you toss in your T.P. Some signs of unhealthy feces need to be seen, says Kit Wong, M.D., a medical oncologist specializing in gastrointestinal cancers at Seattle Cancer Care Alliance in Seattle. Keep in mind that some of these symptoms might be colon cancer—or they might not. That’s why it’s best to see your doctor about any concerning issues.

toilet paper

Look for Blood When You Wipe

Don’t panic. Rectal bleeding is not always related to cancer. Often, it can be due to anal fissures—small tears or cracks in the anus lining—or hemorrhoids, which are veins in your anus and lower rectum that are swollen and bleeding for any number of reasons. Hemorrhoids happen in nearly three out of four adults, and yet in a study of more than 300 patients seen for rectal bleeding, only 3.4% actually had colorectal cancer. However, rectal bleeding CAN be a sign of colon cancer, so you know what to do: Check in with your doctor if it’s new or persists.

chameleon color change

Scan for Color Changes

Colon cancer can cause bleeding in the digestive tract and make your stool a dark brown, maroon, or black. But that coloring could be caused by an ulcer or even what you’ve eaten, says David M. Poppers, M.D., Ph.D., a gastroenterologist at NYU Langone Health in New York City. Other times, bleeding can be so microscopic with colon cancer, it can’t be seen in the toilet—but you might have unexplained anemia. If you have anemia for no discernible reason, he often recommends a colonoscopy, a highly effective colon cancer screening tool, to determine if cancer might be a cause.


Notice Changes in Size and Shape

A quick anatomy lesson: The colon, also known as the large intestine, is the final part of your digestive tract. The lumen is an opening inside it that allows waste to pass through as it exits your body out the rectum. If that area becomes obstructed—for instance, from a large tumor—it can alter your stool’s size and shape. It might look as thin as a pencil. This change alone warrants a visit to your doctor. “Imagine that with a growing tumor in the colon, the lumen is narrower and so the stool is thinner,” Dr. Wong says.

stomach pain doctor

Pay Attention to Pain

Obstructions in your colon don’t just potentially change the size and shape of your BMs—they can cause pain, too, especially if the obstruction is severe and you’re trying to go. If you have persistent, severe abdominal pain and are having difficulty passing stool or gas, a bowel obstruction is a possibility, says Dr. Wong. It’s best to go to the hospital or see your doctor right away, to determine if there’s an obstruction—and what’s causing it.


Note If You're Straining

Yet another problem that a bowel obstruction can cause is constipation. If you have unexplained constipation that won’t resolve, even with over-the-counter solutions, it might be an obstruction. “People have to strain harder because the passage is smaller,” Dr. Wong explains. And again, it might not be colon cancer because constipation has many, many unrelated causes, but if it’s unexplained or coupled with any other signs and symptoms here, you might need a colonoscopy to determine why it’s chronic. Talk to your doctor about options for prepping for that screening tool if you’re struggling to go—they can help.


Track Urgency

On the flip side, diarrhea can be a symptom of colon cancer too, Dr. Poppers says. Cancer can impact the colon’s function, including absorbing water and nutrients, leading to runny stools. But then, just as with so many of these signs in this guide, diarrhea could be caused by something as far removed from cancer as bad sushi. Loose, watery stools that don’t improve with over-the-counter meds, or treatment from your doctor, warrant a follow-up visit and possible colonoscopy, he says.


Pay Attention to Regularity

Maybe you go every day, the same time, and suddenly realize it’s been more than three days for you—and no poo. Or you need a laxative after a few days of no-go for relief—and that’s happening week after week. Maybe you were once regular, but begin alternating between chronic constipation and diarrhea. “There’s not one more likely than the other,” to be related to colon cancer, Dr. Wong says. “Both stool changes are concerning.” Any persistent changes in bowel habits should be reported to your doctor.

almost empty

Understand Tenesmus

Do you feel like you need to go, but no matter how much you strain, you can’t get it all out… no matter how long you stay on the toilet? Or you feel an urge to go but don’t really need to? “The term for that is tenesmus,” says Dr. Poppers. And it can be a sign of colon cancer… or it could be caused by an infection, inflammation (like a condition called proctitis), or gastrointestinal motility disorders. The best thing to do for this symptom is see your doctor to discuss, Dr. Poppers says.

CT scan abdomen

Be Aware of Asymptomatic Colon Cancer

Colon cancer doesn’t always send warning signs via your BMs. “Early colon cancer may produce no symptoms at all, and may be incidentally found during a colonoscopy or a CAT scan of the abdomen or other imaging tests. Or it may be found on standard blood tests,” Dr. Poppers says, or by tests that specifically look for a chemical called carcinoembryonic antigen (CEA) that can be produced by colon cancer. That’s why it’s important to start colonoscopy screenings at 45, or sooner if you're at higher risk thanks to family history or a history of inflammatory bowel disease.

Erin L. Boyle
Meet Our Writer
Erin L. Boyle

Erin L. Boyle, the senior editor at HealthCentral from 2016-2018, is an award-winning freelance medical writer and editor with more than 15 years’ experience. She’s traveled the world for a decade to bring the latest in medical research to doctors. Health writing is also personal for her: she has several autoimmune diseases and migraines with aura, which she writes about for HealthCentral. Learn more about her at erinlynnboyle.com. Follow her on Twitter @ErinLBoyle.