Is foul-smelling poop ever a symptom of cancer?

Some foods — such as beans, eggs and cabbage — are known to cause foul-smelling gas and bowel movements. 

But is foul-smelling poop ever a sign of cancer? If not, what else might be causing it?  

Read on for answers. 

Is foul-smelling poop ever a sign of cancer?

When that is the only issue you have, the answer is likely no. But foul-smelling poop is not one of the classic signs of cancer, either — whether colorectal or some other type. As doctors, we tend to look for other factors when thinking about cancer, such as unexpected weight loss, blood in your stool and changes in bowel habits. 

As with smelly urine, a few studies have shown that specially trained scent hounds can detect chemical compounds in stool samples that are unique to colorectal cancers. But we don’t routinely test for them, and the human nose can’t discern them. 

Then what does cause foul-smelling poop?

The things we eat, the bacteria that live in our gut, and the way they digest our food all contribute to the way poop smells. But certain foods — like meat, fish, eggs, dairy and leafy green vegetables — have higher concentrations than other foods of compounds that smell really bad when broken down. These compounds include: 

• Nitrogen: smells like ammonia 
• Sulfur: smells like rotten eggs 
• Short-chain (or volatile) fatty acids: smells like rancid butter or body odor 
• Thiols (mercaptans): smells like a skunk 

So, you could have really foul-smelling gas or stools and nothing wrong with you, disease-wise. It’s mainly due to what you eat and how it’s broken down.

What other conditions might cause foul-smelling poop?

While cancer doesn’t cause foul-smelling poop, other health issues can. These include: 

Food intolerances

Food intolerance is probably the most common cause of foul-smelling poop. This is when your body lacks the enzymes to digest certain sugars or proteins. When these hit the bacteria of the colon undigested, that can result in some really bad-smelling stools.

Fat malabsorption syndrome

This occurs when your bowel cannot properly process fat. Undigested fats that make their way to the large intestine can cause fatty, greasy stools that smell really foul. 

Celiac disease  

This autoimmune disease affects the gut. It can cause foul-smelling diarrhea. 

Inflammatory bowel disorders

These include Crohn’s disease and ulcerative colitis. Inflammatory bowel disease can give stool a really bad smell because your gut microbiome may be altered. 

Irritable bowel syndrome

This condition has symptoms that come and go, including abdominal cramps, constipation, bad gas and smelly diarrhea. 

Bacterial infections

Most digestive disorders don’t have a specific odor associated with them, but some do. Stool generated by the bacterial infection Clostridium difficile (c. diff). in particular has a very characteristic odor, and once you’ve smelled it, you’re unlikely to forget it. 

Viruses

These include norovirus and rotavirus, which are often called the stomach flu or a stomach bug. In addition to foul-smelling diarrhea, these can also generate very sharp or caustic-smelling vomit.  

So, when should you contact a doctor about stinky poop?

Sometimes, it’s hard to determine if you have a health problem or if you just ate too much broccoli last night. But odor isn’t the only thing to think about when it comes to digestive issues. You have to look at the big picture. 

That’s why we ask questions like, “Do you also have diarrhea?” Or, “Have you taken an antibiotic recently?” Or even, “Have you traveled anywhere lately and been exposed to something?” If the answer to any of those is yes, then that, plus the foul-smelling poop, may point to a problem.

The color of your poop also matters.  

• If it’s bright red or black and tarry, that’s highly suspicious for blood.  
• If it’s light or pasty, that could point to a problem with your liver or pancreas.  

Tan or very light stools are missing the pigments normally found in stool. Among these are bilirubin (an orangey-yellow substance secreted by the liver in bile) and products from the digestion of heme (a type of iron found in animal products). These are the same pigments that make urine yellow and stool brown. 

Stool in shades of brown, green and even yellow is likely nothing alarming. Those colors just represent what you’d expect to see from these pigmented compounds.   

Finally, consider how long your symptoms have lasted. See a doctor about any issues that go on for more than a couple of weeks. 

David Richards, M.D., is a gastroenterologist at MD Anderson.

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