All poop smells, and most of the time a particularly strong odor is completely normal. The smell comes from compounds your gut bacteria produce as they break down food, and certain meals, medications, or temporary digestive changes can make things noticeably worse. That said, a persistent foul smell combined with other symptoms like diarrhea, weight loss, or changes in stool color can occasionally point to a condition worth investigating.
Why Poop Smells in the First Place
The odor of stool is a byproduct of bacterial activity in your intestines. Trillions of microbes in your gut ferment the food you can’t fully digest on your own, and that process generates gases and chemical byproducts. The main culprit behind the classic fecal smell is a compound produced when gut bacteria break down tryptophan, an amino acid found in protein-rich foods like meat, eggs, and dairy. At high concentrations this compound has the characteristic fecal odor most people recognize. Other bacterial byproducts contribute too, including sulfur-containing gases (the “rotten egg” smell) and ammonia, which has a sharp, urine-like quality.
The balance of these compounds shifts depending on what you eat, how quickly food moves through your system, and which bacteria are most active in your gut at any given time. That’s why your stool can smell dramatically different from one day to the next.
Foods That Make Stool Smell Worse
Diet is the single most common reason for unusually smelly poop. Sulfur-rich foods are some of the biggest offenders. Broccoli, cauliflower, cabbage, garlic, onions, and eggs all contain sulfur compounds that gut bacteria convert into hydrogen sulfide gas. The more of these foods you eat, the more raw material your bacteria have to work with.
High-protein diets also intensify stool odor. When your body breaks down large amounts of protein, the process generates ammonia as a byproduct. Research shows that eating a lot of protein raises ammonia levels throughout the body, and the effect extends to your stool. This is worth knowing if you’re on a high-protein or ketogenic diet and have noticed a change.
Fasting and very low-carb diets can shift your body into a state where it burns fat for fuel, which produces ketones. This can give body waste a different, sometimes fruity or acetone-like quality. Alcohol, spicy foods, and artificial sweeteners (especially sugar alcohols found in “sugar-free” products) can also change stool odor by altering how quickly food moves through your gut or by feeding certain bacteria over others.
Antibiotics and Medications
Antibiotics kill off bacteria indiscriminately, including the beneficial species that keep your gut ecosystem balanced. When that balance is disrupted, opportunistic bacteria can overgrow and produce more odorous compounds than usual. The smell typically resolves within a few weeks of finishing the course, as your gut microbiome repopulates. Supplements like iron are also well-known for making stool darker and more pungent.
Infections That Change Stool Odor
Certain gut infections produce distinctly foul-smelling stool that goes beyond the normal range. Giardia, a waterborne parasite you can pick up from contaminated water while traveling or camping, causes smelly, greasy poop that often floats. It typically comes alongside diarrhea, gas, stomach cramps, and nausea. C. difficile, a bacterial infection that commonly follows antibiotic use, produces stool with an unusually strong, sickly-sweet odor that healthcare workers often describe as unmistakable.
Both infections tend to cause diarrhea lasting more than a few days, which is a key distinguishing feature. If your stool suddenly smells much worse and you also have watery diarrhea, especially after recent travel or antibiotic use, an infection is a real possibility.
Malabsorption and Fatty Stools
When your body can’t properly absorb fats from food, those fats pass through to your stool, producing a condition called steatorrhea. The result is pale, oily, foul-smelling poop that may be difficult to flush. It often leaves a greasy residue in the toilet bowl.
Several conditions can cause this. Celiac disease damages the lining of the small intestine, reducing its ability to absorb nutrients. Chronic pancreatitis and cystic fibrosis both impair the pancreas’s ability to produce the digestive enzymes needed to break down fat. Even small intestinal bacterial overgrowth (SIBO), where bacteria colonize the wrong part of the gut, can interfere with fat absorption.
The hallmark of malabsorption is that it’s persistent. If your stool looks greasy, floats consistently, and smells unusually rancid for weeks rather than days, your body may not be digesting fat properly. Doctors can test for this with a stool sample that measures how well your pancreas is functioning. Normal pancreatic function produces enzyme levels above 500 micrograms per gram of stool, while levels below 200 suggest significant insufficiency.
What the Color and Texture Tell You
Smell alone rarely signals a serious problem, but smell combined with a visual change is more meaningful. According to MD Anderson Cancer Center, bright red or black tarry stool is highly suspicious for blood in the digestive tract. Light-colored or clay-like stool can indicate a problem with the liver or pancreas, since bile is what gives stool its normal brown color. Greasy, floating stool points to fat malabsorption.
The consistency matters too. Stool that’s consistently loose or watery alongside a new, strong odor suggests your gut is either fighting an infection or struggling to absorb nutrients. Formed stool that simply smells bad after a big steak dinner is almost certainly just your bacteria doing their job.
When Smell Alone Isn’t the Problem
Doctors don’t typically consider foul-smelling stool a red flag on its own. What they look for are accompanying changes: unexpected weight loss, blood in the stool, persistent diarrhea, or a sustained shift in bowel habits. If you’re experiencing any of those alongside noticeably worse-smelling stool, that combination is worth bringing up.
A useful rule of thumb is the two-week mark. A few days of unusually smelly poop after a dietary change, a round of antibiotics, or a stomach bug is expected. If the change persists for more than a couple of weeks without an obvious explanation, it’s reasonable to get it checked out. The evaluation is usually straightforward, often starting with a stool sample and basic blood work to look for signs of infection, inflammation, or malabsorption.
Simple Ways to Reduce Stool Odor
If your smelly poop is a nuisance but not accompanied by other symptoms, diet adjustments are the most direct fix. Cutting back on sulfur-heavy vegetables, red meat, and heavily processed foods for a week or two can make a noticeable difference. Adding more fiber-rich whole grains and fruits feeds the types of bacteria that produce less odorous byproducts.
Staying well-hydrated helps keep digestion moving at a normal pace. When stool sits in the colon too long (constipation), bacteria have more time to ferment it, which intensifies the smell. Fermented foods like yogurt, kefir, and sauerkraut can help restore microbial balance, especially after a course of antibiotics. If you’re on a high-protein diet, gradually increasing your fiber intake gives your gut bacteria a more diverse mix to work with, which tends to reduce the ammonia-heavy odor that comes from digesting protein alone.