Why Does My Poop Smell So Bad? Causes and Red Flags

Why does my poop smell so bad? Stool smells because gut bacteria break down what you did not digest, and the gases and compounds they release include ammonia and sulfur compounds^[1]. Meat-heavy meals, antibiotics and stomach infections are the usual triggers^[2][3][4]. A smell that lasts, or comes with greasy stools, weight loss or diarrhea, is the one to look into.

This page explains what produces the odor, which smells point where (and which do not), what you can try, and the specific signs that need a doctor. If the shape or texture of your stool concerns you more than the smell, start with our Bristol Stool Chart guide.

Get medical help promptly if your smelly stool comes with black, tarry or bloody stool, severe belly or rectal pain, signs of dehydration, a high fever, or diarrhea that lasts more than 2 days. These can signal infection, bleeding or malabsorption and need a clinician, not home care^[4].

What makes poop smell in the first place?

The odor comes mostly from bacteria in your large intestine. A review of body odor describes microbes producing ammonia, volatile sulfur compounds and trimethylamine. How much builds up depends on diet, your microbiota and how well your liver, intestines and kidneys work^[1].

Protein is the best-documented fuel. In a controlled feeding study, five healthy men ate five diets for 10 days each, from no meat to 600 g of meat a day. Fecal sulfide climbed from 0.22 to 3.38 mmol/kg across those diets, tracking meat intake^[2]. In stool cultures from the same work, protein digestion produced sulfide and ammonia together^[2]. The study was small, and 600 g of meat is far above a typical day, but the direction is clear.

The sulfur gas involved is hydrogen sulfide. Sulfur-containing amino acids drive it: in lab slurries of healthy human stool, adding the amino acid cysteine raised hydrogen sulfide output by about 1,557%, while inorganic sulfate had little effect^[5]. The same gas explains why farts smell like sulfur and why some people get sulfur burps.

What do different poop smells mean?

Most smells do not identify a cause on their own. This table pairs the smells and stool features that sources do describe with the usual explanations.

Some smells are missing from that table on purpose. We found no clinical source that links a sweet, fruity, burnt-hair or fishy stool smell, or a bowel movement that smells like death, to a specific cause, so we do not assign one. The same goes for stool that does not smell at all. Mogilnicka and colleagues list ammonia, sulfur compounds and trimethylamine as the microbial odorants, but not which one a given person is smelling^[1].

Why does poop smell like sulfur or rotten eggs?

Rotten-egg or sewer-like stool is almost always hydrogen sulfide from bacterial fermentation of protein, which is why it follows meat-heavy meals. Meat supplies sulfur amino acids, and the 600 g meat diet above produced about 15 times the fecal sulfide of the meat-free diet^[2].

The microbes behind it include sulfate-reducing bacteria such as Desulfovibrio and Bilophila. You cannot easily diet them away: in a pilot crossover study of four healthy volunteers eating high- and low-sulfur-amino-acid diets for 10 to 14 days, the abundance of these bacteria did not change, and each person's baseline stayed stable over time^[9]. The authors concluded that diet alone may be insufficient to target them quickly^[9].

That leaves the amount of fuel, and the fiber that competes for it. In the same lab-slurry work, fermentable fibers cut hydrogen sulfide output sharply: resistant starch by 89% and fructo-oligosaccharides (FOS) by 82%, against under 35% for psyllium^[5]. That is bench data, not a trial in people, so treat resistant starch and FOS as reasonable things to test, not a proven fix.

Why does my poop smell bad and look greasy or pale?

Foul, greasy, hard-to-flush stool points to fat that was not absorbed (steatorrhea). Fat malabsorption can come from exocrine pancreatic insufficiency, bile acid disorders or intestinal disease, and it can bring weight loss and fat-soluble vitamin deficiencies^[6]. NIDDK lists "loose, greasy, foul-smelling bowel movements" with bloating, gas and appetite change as malabsorption symptoms^[4].

Three causes come up most often:

• Pancreatic insufficiency. The AGA lists steatorrhea, weight loss, bloating, excessive flatulence and fat-soluble vitamin deficiency as its features. A fecal elastase below 100 μg/g of stool gives good evidence of it, and 100 to 200 μg/g is indeterminate^[10].
• Celiac disease. Symptoms can include diarrhea, steatorrhea, weight loss, bloating and flatulence, though many people have none. Blood tests and biopsy must be done while you still eat gluten, so do not cut it out first^[11].
• Giardia. CDC describes "smelly, greasy poop that can float" with gas and cramps. Symptoms typically start 1 to 2 weeks after infection and last 2 to 6 weeks^[7].

Pale stool has its own causes, covered in our guide to pale or clay-colored stool, and the floating stool guide explains the gas-versus-fat question.

Can a smell tell you it is C. diff?

Not reliably. C. difficile infection happens when the bacterium produces a toxin that causes diarrhea and inflammation of the colon^[12]. Antibiotics are a recognized risk because they disrupt the gut flora^[4][3].

There is anecdotal evidence that C. difficile stool smells distinctive, but a 2019 study of 53 infected and 53 uninfected patients with diarrhea had to use lab instruments to find the difference. Seven volatile compounds separated the groups, including indole and p-cresol (the compound behind the p-cresol page), with accuracy scores (ROC areas) of 0.73 to 0.85^[13]. That is promising for future point-of-care tests, not a reason to trust your nose^[13]. Diagnosis needs a stool test, and the ACG guideline covers how it is confirmed and treated^[12].

If you have had antibiotics in the past few weeks and now have watery, foul diarrhea with cramps or fever, call your doctor rather than waiting.

Is foul-smelling poop a sign of cancer?

Smell alone is not a recognized warning sign. A systematic review of studies in primary care ranked the features that best predict colorectal cancer: a palpable rectal or abdominal mass, rectal bleeding with weight loss, iron-deficiency anemia, blood mixed with stool, dark rectal bleeding, and a change in bowel habits^[14]. Stool odor was not among the features listed in that review^[14].

So the smell itself is not the warning sign. Blood, black stool, a lasting change in how often you go, unexplained weight loss or anemia are. Our black stool guide explains the harmless causes, such as iron and bismuth, and the signs of bleeding.

What can you try at home?

Start with a two-week log of what you ate, any new medicines, and what your stool looked and smelled like. The log is what a clinician will want to see, and it can show whether smell tracks specific foods.

1. Check the obvious triggers. Count high-meat days; the feeding study above shows the dose-response link^[2].
2. Review new medicines. Antibiotics reduce gut microbial diversity and alter its metabolic activity, which can cause antibiotic-associated diarrhea^[3]. Do not stop a prescribed drug without asking your prescriber.
3. Test dairy. If bloating and loose, smelly stool follow milk or ice cream, a short low-lactose trial with your clinician is reasonable^[8]. Lactose intolerance varies with dose and microbiome.
4. Try more fermentable fiber gradually. The lab evidence above is encouraging but untested in people^[5].
5. Do not self-treat for weeks. If the smell has no clear food or medicine cause after a couple of weeks, get it checked. The farts guide covers odor remedies for gas.

Constant smell with IBS-type symptoms deserves a conversation too. The ACG IBS guideline suggests celiac blood tests and fecal calprotectin in people with IBS and diarrhea, and a limited low-FODMAP trial^[15]. NIDDK also lists SIBO among digestive-tract problems that can cause chronic diarrhea^[4].

When should you see a doctor?

Talk to a doctor right away for stool that is black and tarry or contains red blood or pus, severe abdominal or rectal pain, signs of dehydration (extreme thirst, dark urine, dizziness), frequent vomiting, or a change in mental state^[4].

Adults should also get advice for diarrhea lasting more than 2 days, a high fever, or six or more loose stools a day. People who are pregnant, over 65, taking antibiotics or immunocompromised are more likely to run into trouble and should stay in touch with their doctor^[4].

Book a routine visit if a foul smell lasts more than a couple of weeks with no clear food or medicine cause, if stools are persistently greasy or pale, or if you are losing weight without trying. Those are the features that sources tie to fat malabsorption, which is testable^[10][6].

This article is for general educational purposes and is not medical advice. Stool odor alone cannot diagnose a condition, and a nose is not a test. Talk to a qualified healthcare professional before changing your diet or medicines because of a change in your stool, particularly if you are pregnant, immunocompromised, taking prescription medication, or managing a diagnosed condition.