Is Foul-Smelling Poop a Sign of Cancer?

A Keck Medicine of USC gastrointestinal surgeon explains when changes in your stool odor could warrant concern.

If you’re worried about a significant change in the smell of your stool, you may feel too embarrassed to discuss the change with your health care provider. There are instances, however, when seeing a doctor is a good idea.

While food and drinks can temporarily cause changes in your stool, if the changes in your stool persist for days or even weeks, it may warrant concern.

Changes in stool odor, caliber and/or color could indicate a more serious development like cancer, says Sharon Shiraga, MD, an upper-gastrointestinal surgeon with Keck Medicine of USC who specializes in gastrointestinal conditions. She explains when you should consult a doctor.

When are changes in stool odor normal?

Most foul-smelling stool has non-cancerous causes. These can include eating a lot of highly processed foods or eating foods you don’t normally eat. They can also include conditions like pancreatic insufficiency (when the pancreas doesn’t produce enough digestive enzymes, causing poor digestion and malabsorption of nutrients) or bacterial infections.

The key to detecting problematic stool odor changes lies in distinguishing between temporary variations and potentially significant alterations.

When should you worry about changes in stool odor?

Temporary changes in stool odor caused by dietary factors are usually not a concern. “That adventurous meal you ate where you saw some stool changes afterward? That is normal,” Dr. Shiraga says.

Stool changes only become concerning if they continue over time despite returning to your normal diet. Although it happens infrequently, foul-smelling dark stool can occasionally indicate bleeding in the gastrointestinal tract caused by the development of a gastrointestinal cancer like colon cancer or stomach cancer.

This cancer-related bleeding produces a distinctive odor: a “rotten blood” smell that’s distinctly metallic and appears alongside inky, dark, black stool that continues for days or weeks. “When your stool has blood in it, it becomes darker and smells bad,” Dr. Shiraga says.

She points out that gastrointestinal cancers have varying prevalence rates. Colon cancer represents the third most common cancer overall, stomach cancer affects 3% of the U.S. population and esophageal cancer affects 4.3%. Another type, small bowel cancer, is very rare but highly aggressive when it occurs.

“Cancer-related symptoms are persistent,” Dr. Shiraga emphasizes. “They continue for days or weeks regardless of your diet. If you have bleeding from cancer, it won’t improve just because you ate tacos today or Thai food tomorrow.”

Be aware of stool changes

It’s important to stay alert about any changes in your stool. Understand what your normal stool patterns are so that you recognize any significant deviations. Dr. Shiraga recommends paying attention to the size, color and consistency of your stool.

If you notice any of the following signs, you should consult a doctor:

• Changes in your stool odor that persist for days or weeks despite dietary modifications
• Dark, black, bad-smelling stool
• Unintentional weight loss or unexplained appetite loss
• Persistent changes in stool thickness

“If you notice any of these changes persistently, you need to talk to your health care provider as soon as possible,” Dr. Shiraga advises. Consider recording daily notes about your observations to share with your doctor.

How do doctors evaluate concerning stool changes?

Doctors will evaluate your stool characteristics. They will also consider your personal and family medical history, any associated health symptoms and your overall health status.

They may recommend testing, such as a blood test to detect any hidden blood in your stool or stool tests that can identify bacterial infections. In some cases, an endoscopic exam is recommended.

Don’t be shy about sharing your concerns

Most foul-smelling stools have simple explanations like dietary factors or digestive conditions. However, don’t let embarrassment prevent you from discussing persistent symptoms with your health care provider.

“Be your own advocate,” Dr. Shiraga says. “Don’t be shy about having an open discussion with your primary care doctor. Remember that medical professionals have seen it all, so there is no need to feel embarrassed.”

Importantly, stay vigilant about monitoring yourself for any changes. “Pay attention to what’s normal for you,” Dr. Shiraga says. Keep any eye out for stool odor changes that last for days or weeks.

And maintain perspective, she says. While cancer can occasionally cause stool odor changes, this occurs less frequently than many people fear. Most diagnostic evaluations reveal treatable causes.

Follow recommended colorectal cancer screening protocols. By understanding what to monitor and when to seek care, you can confidently evaluate stool changes and take appropriate action when needed, knowing that most concerns have simple explanations and effective treatments.

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