A leading gastroenterologist shares the most common causes behind an odd-smelling stool, and when it might signal a health issue.
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1. You’ve changed up your diet
“Diet is one of the most common and benign reasons for stool odor changes,” Dr. Zhang says. “Foods high in sulfur, like red meat, eggs, garlic, onions, and cruciferous vegetables (such as broccoli, cauliflower, or cabbage) can create more potent odors as they’re broken down.”
High-protein diets and heavily processed foods can also make stools smell more pungent, she adds.
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2. You’re drinking more alcohol
Alcohol disrupts your microbiome and digestion, according to Dr. Zhang. “This can noticeably alter stool odor both for the next day and for the long term,” she says.
Some types of alcohol also contain sulfites—notably wines, ciders, and beers. These sulfur-based preservatives produce more hydrogen sulfide during digestion, contributing to smellier poop.
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3. You’re stressed
Stress is a major culprit behind gut and bowel movement changes. “Stress can speed up or slow down how food moves through your intestines, altering bacterial fermentation or stool composition,” Dr. Zhang says. (Poor sleep can have these effects, too!)
These shifts can lead to incomplete food digestion, allowing more foul-smelling compounds like hydrogen sulfide to form in your bowel movements, according to 2023 research published in The Journal of Physiology.
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4. You’re dehydrated
Dehydration is a major cause of constipation, and when your stool is harder and more concentrated, its smell may intensify as well, Dr. Zhang explains. “Generally speaking, constipation intensifies stool odor by allowing increased bacterial fermentation,” because the stool sits in your colon for longer.
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5. You have an infection
Some infections can cause distinctive stool odors. “One example is Clostridioides difficile (C. diff), which often follows antibiotic use,” Dr. Zhang says. “It produces a distinctively foul, almost sweet smell that’s typically accompanied by watery diarrhea, cramping, and fever.”
Some antibiotics taken to treat an infection can, once again, alter the microbial makeup within your digestive system and influence a shift in how waste smells.
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6. You ate a rich meal
Dr. Zhang says an excessively fatty meal can overwhelm the digestive system and lead to a “fatty stool,” also known as steatorrhea. “This kind of stool smells particularly rancid or foul due to undigested fat.” It may also look pale, greasy, and stick to the toilet bowl.
While it’s less common, persistent fatty poops may indicate a problem with how your body digests and absorbs fat. “This is often due to pancreatic disease,” Dr. Zhang says. (Here are other causes of steatorrhea.) “The first step, if you think you might have problematic steatorrhea, is to try a few low-fat and vegetable-based meals and see if the smell goes away.”
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7. You may have a gastrointestinal issue
A variety of digestive and non-digestive issues can cause a change in stool odor, Dr. Zhang says. “Usually, there are other symptoms, and a change of stool odor is very rarely the only sign of the disease.”
Still, she says that one smell that gastroenterologists pay particular attention to is the odor of blood in the stool. “If the bleeding is from higher up in the digestive tract, such as the stomach or small intestine, the blood gets digested, producing sulfur-containing compounds that smell fishy, rotten, or just distinctly unpleasant,” she says. “If bleeding is lower in the gut, like in the colon or rectum, the smell is more metallic, like iron or nosebleed.”
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Does foul-smelling poop indicate cancer?
“In rare cases, yes,” Dr. Zhang says. “Cancers of the gastrointestinal tract, especially stomach or colorectal cancer, can cause blood loss and give off the distinctive smell of digested or undigested blood,” she explains. “Pancreatic cancer can cause the smell of steatorrhea (fatty stool), which many people describe as rancid or repulsive.”
That said, cancer usually presents with other warning signs, such as unintentional weight loss, fatigue, black tarry stool, blood in the stool, or a persistent change in bowel habits, Dr. Zhang says. “Odor alone is not typically how cancer is first detected,” she says, “but it can be part of a broader symptom picture.”
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When to see a doctor for smelly poop
“If your stool smells different for a day or two after a heavy or unusual meal, that’s usually nothing to worry about,” Dr. Zhang says. “But if the change lasts more than a few days or comes with other symptoms, it’s best to get checked.”
She advises that you see a doctor if you experience any of these red flags:
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Blood in the stool
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Black tarry stool (also known as ‘melena,’ a dark, shiny, and sticky stool that is foul-smelling)
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Persistent diarrhea or constipation
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Unexplained weight loss
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Ongoing fatigue
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Greasy stool that sticks to the toilet bowl (steatorrhea)
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Abdominal pain, cramping, or bloating
The bottom line
Everyone has their own “normal” poop smell, but many factors can cause not-so-nice odor changes. Most causes are harmless, but if the smell doesn’t go away or comes with other symptoms, it’s important to get checked out by your doctor.
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