All stool smells, but an unusually foul odor usually comes down to what you ate, how your gut bacteria processed it, or occasionally a digestive condition that needs attention. The smell is created by specific chemical byproducts of bacterial fermentation in your large intestine, and certain foods, medications, and health conditions can shift that process into overdrive.
What Actually Creates the Smell
Your colon is home to trillions of bacteria that break down whatever your small intestine didn’t fully absorb. That fermentation process produces gases and chemical compounds, and several of them are intensely odorous. The primary culprit is a compound called skatole, produced when gut bacteria metabolize the amino acid tryptophan (found in most protein-rich foods). The chemist who first isolated skatole in the 1800s described it as having “an extremely unpleasant fecal and fascinating odor.” Skatole is, quite literally, the signature smell of feces.
Hydrogen sulfide is the other major player. It’s the same gas responsible for the rotten-egg smell of hot springs and volcanic vents. Bacteria in your bowel convert sulfur from food into hydrogen sulfide through fermentation, and more sulfur in your diet means more of this gas in your stool and flatulence. A related compound called indole adds its own sharp, pungent note to the mix. Nitrogen-containing byproducts from protein digestion contribute an ammonia-like edge. The final smell of any bowel movement is a blend of all these compounds in varying proportions, which is why the odor changes from day to day.
Foods That Make It Worse
Diet is the single biggest variable in how your stool smells on any given day. Sulfur-rich foods are the most reliable trigger for especially foul odor. These include eggs, cheese, whole milk, ice cream, mayonnaise, and soy milk. Cruciferous vegetables like broccoli, cabbage, cauliflower, and Brussels sprouts are also high in sulfur compounds. When gut bacteria ferment that extra sulfur, hydrogen sulfide production spikes.
High-protein meals are another common cause. Meat, fish, eggs, and dairy all contain high concentrations of nitrogen and sulfur compounds that produce particularly bad-smelling byproducts when broken down by colonic bacteria. If you’ve recently increased your protein intake, whether through diet changes or protein supplements, the shift in odor can be dramatic. The excess protein that isn’t absorbed in the small intestine reaches the colon, where bacteria ferment it into ammonia and other pungent gases.
Garlic, onions, and spicy foods round out the usual suspects. Alcohol can also change stool odor by altering gut motility and the balance of intestinal bacteria.
Food Intolerance and Poor Absorption
Food intolerance is probably the most common cause of persistently foul-smelling stool. When your body lacks the enzymes to digest certain sugars or proteins, those undigested molecules pass into the colon, where bacteria feast on them and produce excessive gas and odor. Lactose intolerance is a classic example: undigested milk sugar ferments aggressively in the colon, often producing bloating, cramping, and notably smelly stools and gas.
A more serious version of this is fat malabsorption, known medically as steatorrhea. When your body can’t properly digest fat, whether because the pancreas isn’t producing enough digestive enzymes or because bile flow is blocked, undigested fat passes through to your stool. Fatty stools have a distinct presentation: they tend to be bulky, loose, greasy, foamy, light-colored (like clay), and unusually smelly. They often float and can be hard to flush. Conditions that cause this include celiac disease, Crohn’s disease, chronic pancreatitis, and small intestinal bacterial overgrowth (SIBO). If your stool consistently looks pale, greasy, and smells worse than usual, that pattern is worth bringing to a doctor.
Antibiotics and Medications
Antibiotics are a frequent cause of temporary changes in stool odor. They kill off beneficial gut bacteria along with harmful ones, disrupting the balance of your intestinal ecosystem. With fewer “good” bacteria competing for resources, different fermentation patterns take over, often producing worse-smelling results. This usually resolves on its own after you finish the antibiotic course and your normal gut bacteria repopulate.
In some cases, antibiotics wipe out enough beneficial bacteria that harmful species multiply out of control. The most concerning of these is a bacterium called C. diff, which causes an infection with very distinctive stool. People with C. diff diarrhea often describe the odor as unusually strong and oddly sweet, likely because the infection increases bile acid levels in stool. C. diff infections typically develop during or shortly after antibiotic use, and they require treatment. Iron supplements and certain vitamins can also change stool odor and color noticeably.
Infections and Gut Bugs
Gastrointestinal infections from bacteria, viruses, or parasites commonly produce foul-smelling diarrhea. Food poisoning from bacteria like Salmonella or E. coli often causes sudden-onset watery, smelly stool along with cramping and sometimes fever. Parasitic infections like Giardia, which you can pick up from contaminated water, are particularly notorious for producing greasy, sulfurous-smelling stools that persist for weeks if untreated.
Most acute stomach bugs resolve within a few days. Stool that remains unusually foul-smelling for more than a week or two, especially with ongoing diarrhea, suggests something beyond a typical short-lived infection.
Signs Something More Serious Is Going On
Bad-smelling stool by itself, especially after a big steak dinner or a plate of eggs, is almost always harmless. The smell becomes more meaningful when it shows up alongside other changes. Pay attention if you notice any of the following patterns persisting:
- Black or very pale stools: Black stool can indicate bleeding higher in the digestive tract, while pale or clay-colored stool suggests a problem with bile production or flow.
- Blood in the stool: Visible red blood or a positive test for hidden blood warrants investigation.
- Unexplained weight loss: Losing weight without trying, combined with foul stool, can point to malabsorption conditions where your body isn’t extracting nutrients properly.
- Persistent greasy, floating stools: This pattern suggests fat malabsorption and points toward pancreatic, liver, or small intestine problems.
- Fever, chills, or severe abdominal pain: These alongside foul stool suggest an active infection or inflammatory condition.
Any of these combinations deserves medical evaluation. For fat malabsorption specifically, a doctor can run a fecal fat analysis, collecting stool samples over 24 to 72 hours to measure fat content and determine whether digestion is working properly.
Simple Ways to Reduce Stool Odor
If your stool smells worse than usual and you’re otherwise feeling fine, diet is the first place to look. Cutting back on sulfur-heavy foods like eggs, cruciferous vegetables, and dairy for a few days often makes a noticeable difference. If you’re eating large amounts of protein, especially from supplements, scaling back can reduce the ammonia-like odor that comes from excess protein fermentation in the colon.
Staying hydrated helps keep digestion moving at a normal pace. Stool that sits in the colon longer undergoes more bacterial fermentation, which intensifies the smell. Eating enough fiber keeps transit time reasonable and feeds beneficial bacteria that compete with the sulfur-reducing species responsible for the worst odors. Probiotic-rich foods like yogurt, kefir, and fermented vegetables can help restore bacterial balance, particularly after a round of antibiotics. If you suspect a specific food intolerance, an elimination diet, where you remove one suspect food at a time for two to three weeks, is the most practical way to identify the trigger.