Stool that smells like cat urine almost always points to an excess of ammonia or ammonia-like nitrogen compounds in your gut. Cat pee is notorious for its sharp, pungent odor precisely because it is rich in ammonia and urea, so when your stool gives off a similar smell, the overlap is chemical, not coincidental. The causes range from something as simple as a recent shift in your diet to less common conditions involving your liver, kidneys, or a specific enzyme deficiency. Sometimes, though, the issue is not your stool at all but rather a distortion in the way your nose processes smells.
The Chemistry Behind That Sharp, Ammonia-Like Smell
Normal stool contains a cocktail of volatile compounds, most of which are produced by the trillions of bacteria living in your large intestine. These microbes break down the leftover food your small intestine didn’t absorb, and the byproducts include gases and small molecules that give stool its characteristic odor. Among the key offenders are hydrogen sulfide (the “rotten eggs” smell), short-chain fatty acids, and ammonia. In laboratory analyses of human fecal gases, ammonia has been measured at concentrations below 100 parts per billion, alongside hydrogen sulfide at roughly 5 to 26 ppb and methyl mercaptan at 2 to 15 ppb.1Journal of Health Science. Analysis of Malodorous Substances of Human Feces Under ordinary circumstances, ammonia is present in low enough amounts that it blends into the general stool smell rather than dominating it.
When something tips the balance and ammonia (or a related compound called trimethylamine) spikes, the overall odor profile shifts toward that pungent, urine-like quality. Your gut bacteria are the primary factory for these nitrogen-based molecules. They generate ammonia when they ferment undigested protein, and they produce trimethylamine when they metabolize certain dietary compounds like choline and carnitine. The accumulation of these odorous molecules can result from what you eat, the specific mix of bacteria colonizing your gut, or reduced function of the liver, intestines, or kidneys.2PubMed Central. Microbiota and Malodor-Etiology and Management
High-Protein Diets Are the Most Common Culprit
If you’ve recently upped your protein intake, that alone can explain the cat-pee smell. When you eat more protein than your small intestine can fully absorb, the excess reaches the colon, where bacteria go to work fermenting it. A major end product of that fermentation is ammonia. Research on dietary protein and gut chemistry has shown that high-protein diets result in large-bowel ammonia concentrations roughly twice as high as low-protein diets.3PubMed. Independent effects of fiber and protein on colonic luminal ammonia concentration That doubling is enough to change what you notice when you flush.
This does not mean protein is bad for you. It means your colon is doing exactly what it is designed to do with undigested material. The effect tends to be most noticeable when someone dramatically increases protein intake over a short period, like starting a new high-protein diet, adding protein shakes, or eating large quantities of red meat, eggs, or fish. It also tends to be more pronounced when fiber intake is low, because fiber feeds a different set of bacteria that produce less ammonia and more of the short-chain fatty acids associated with a milder stool odor. Adding fiber-rich foods alongside the protein can blunt the ammonia spike noticeably.
Asparagus, garlic, cruciferous vegetables like broccoli and cauliflower, and certain supplements (especially those containing sulfur-bearing amino acids like methionine and cysteine) can compound the problem. The sulfur compounds these foods introduce don’t smell exactly like ammonia, but when mixed with elevated ammonia levels, the combination can produce a stool odor that reads as harsh and urine-like rather than just unpleasant in the usual way.
Trimethylaminuria and the Fish-Odor Connection
Trimethylamine, or TMA, is a volatile amine with a strong, fishy, sometimes urine-like odor. Everyone produces some TMA in the colon when gut bacteria break down choline, lecithin, and carnitine from foods like eggs, liver, legumes, and saltwater fish. Normally, your liver converts TMA into an odorless compound before it can build up. It does this using a specific enzyme called FMO3. This conversion is efficient enough that most people never notice TMA in their body odor, breath, or stool.
In a condition called trimethylaminuria, sometimes known as fish odor syndrome, an inherited deficiency in FMO3 means the liver cannot adequately convert TMA to its odorless form.4PubMed Central. A review of trimethylaminuria: (fish odor syndrome) The result is that TMA accumulates and gets excreted in sweat, breath, urine, and stool.5PubMed. Treatments of trimethylaminuria: where we are and where we might be heading People with trimethylaminuria often describe their body odor as fishy, but depending on concentration and the mix of other compounds, it can also register as strongly ammonia-like or, yes, cat-pee-like. The overlap makes sense because TMA and ammonia are chemically related: both are nitrogen-based small molecules with sharp, pungent smells that our noses process in similar ways.
Trimethylaminuria is rare, and most people who notice an occasional ammonia-like stool smell do not have it. But if the smell extends beyond stool to your sweat, breath, or urine, and it persists regardless of dietary changes, it’s worth mentioning to your doctor. A simple urine test can measure TMA levels relative to its odorless metabolite and give a clear answer. Dietary management, specifically reducing intake of choline-rich and carnitine-rich foods, is the primary treatment, since there is currently no drug that replaces the missing enzyme function.
When Your Gut Bacteria Are Out of Balance
You don’t need a genetic condition for your gut to overproduce ammonia or TMA. The specific composition of your gut microbiota matters enormously. Different bacterial species generate different metabolic byproducts, and shifts in that microbial community can change what your stool smells like. A round of antibiotics, for example, can wipe out certain bacterial populations while allowing others to flourish. If the surviving bacteria happen to be heavy ammonia or sulfur producers, the odor shift can be dramatic and sudden.
Other disruptors include prolonged illness, major dietary changes, heavy alcohol use, and chronic stress, all of which alter the gut microbial landscape in ways that can tip the balance toward more pungent metabolic byproducts.2PubMed Central. Microbiota and Malodor-Etiology and Management In most cases, the smell resolves as the microbiota rebalances over days to weeks. Fermented foods, prebiotic fibers, and simply returning to your usual diet can accelerate that process. If an antibiotic course triggered it, the timeline tends to be a few weeks before things normalize, though individual variation is wide.
Kidney and Liver Problems Worth Knowing About
The liver and kidneys are your body’s main filtration and detoxification systems, and both play roles in clearing ammonia and other nitrogen waste products from your blood. When either organ is underperforming, those waste products can accumulate throughout the body, including in the gut.
In kidney disease, the body struggles to excrete urea and other nitrogen-containing molecules through urine. Some of that excess gets shunted to the colon, where bacteria convert it back into ammonia. Research on chronic kidney disease has shown that fecal matter from affected subjects releases larger amounts of sulfur-containing compounds like dimethyl disulfide and dimethyl trisulfide, which are intensely pungent.6ScienceDirect. Exhaled breath and fecal volatile organic biomarkers of chronic kidney disease The resulting stool odor can take on a sharply chemical, ammonia-like quality that is distinctly different from normal variation.
Liver dysfunction can produce similar effects through a different route. A healthy liver converts ammonia (a toxic byproduct of protein metabolism) into urea, which the kidneys then excrete. When liver function declines, ammonia levels in the blood rise. Some of that ammonia finds its way into the gut. People with advanced liver disease sometimes notice that their stool, breath, and sweat all take on an unusual odor, sometimes described as musty or ammonia-like.
To be clear, a one-time cat-pee smell from your stool almost certainly does not indicate kidney or liver disease. These organ-related causes usually come with other symptoms: fatigue, swelling in the legs, dark urine, jaundice, nausea, or unexplained itching. The smell change, when it is organ-related, also tends to be persistent rather than intermittent. Still, if you notice a sustained shift in stool odor alongside any of these other signs, it’s reasonable to ask your doctor for basic bloodwork checking kidney and liver function.
Sometimes the Problem Is Your Nose, Not Your Stool
Here’s a possibility that catches people off guard: the stool hasn’t changed at all, but the way you perceive its smell has. Parosmia is a condition in which your brain misinterprets real odors. Things that once smelled normal suddenly smell wrong, often registering as chemical, rotten, or reminiscent of cat urine. Parosmia has become far more common in recent years because it is one of the lingering effects of COVID-19 infection.
Research on parosmia has found that the distortions are mainly triggered by specific molecular families, particularly thiols and pyrazines, which are abundant in everyday smells including cooked food, coffee, and yes, stool.7PubMed Central. Parosmia and Phantosmia: Managing Quality Disorders In qualitative studies of people living with post-COVID smell distortions, stool odor was a recurring topic. Participants reported their stool smelling like cat food, rotten eggs, or an unfamiliar “sickly sweet” smell. Some people found that stool had become less offensive than usual while food had become more disgusting, suggesting a wholesale rearrangement of how their brain categorizes odors rather than any change in what the body is actually producing.8PLOS ONE. Altered smell and taste: Anosmia, parosmia and the impact of long Covid-19
If your stool suddenly smells like cat pee and you’ve had a recent respiratory illness, or if other familiar smells have also changed (coffee tastes burnt, onions smell metallic, perfume smells like chemicals), parosmia is a strong candidate. The good news is that it usually improves over months as the olfactory neurons regenerate, though the timeline is frustratingly variable. Smell training, which involves deliberately sniffing a set of distinct scents for a few minutes each day, is the most commonly recommended intervention and has some evidence behind it.
Anxiety and Heightened Smell Sensitivity
There’s another layer to this that is rarely discussed but genuinely relevant. People with higher levels of anxiety tend to have a lower threshold for detecting potentially threatening smells in their environment.9PubMed. Generalized and social anxiety symptoms are differentially associated with common and social odor sensitivity If you’re going through a period of heightened stress or anxiety, you may literally be picking up on odors that were always present in your stool but that you previously filtered out. The ammonia and sulfur compounds have always been there. What changed is that your brain is now flagging them as noteworthy.
This is not the same as saying the smell is imaginary. You are genuinely detecting real volatile compounds. But the reason you’re suddenly noticing them, fixating on them, and categorizing them as alarming (“this smells like cat pee, something must be wrong”) may have more to do with your nervous system’s current sensitivity setting than with any change in your digestive health. If the smell concern appeared alongside a period of general health anxiety, and nothing else about your digestion has changed, this is worth considering before pursuing extensive medical workups.
Practical Steps to Narrow Down the Cause
Because the possible explanations range from mundane to medical, a bit of detective work can save you unnecessary worry. Start with what you’ve been eating. If you recently increased your protein intake, started a new supplement, or ate a large amount of asparagus, eggs, or fish, the simplest explanation is dietary. Try pulling back on protein for a few days and adding more fiber-rich foods. If the smell resolves, you have your answer.
If dietary changes don’t help, think about timing. Did the smell start after an illness, an antibiotic course, or a period of significant stress? Post-antibiotic dysbiosis and post-viral parosmia are both common and both self-limiting, though they can take weeks to months to fully resolve. Pay attention to whether other smells have also changed, which would point more toward a nose issue than a gut issue.
If the ammonia-like odor persists for more than a few weeks without an obvious dietary or illness-related explanation, and especially if it extends to your urine, sweat, or breath, see your doctor. The relevant initial tests are straightforward:
- Urine TMA test: Checks for trimethylaminuria by measuring the ratio of TMA to its odorless metabolite in your urine.
- Basic metabolic panel: Screens kidney function through creatinine and blood urea nitrogen levels.
- Liver function tests: Measures enzymes and bilirubin to assess whether the liver is clearing waste products effectively.
- Stool analysis: Can identify malabsorption, bacterial overgrowth, or infection that might be driving unusual fermentation patterns.
None of these tests are invasive or unusual. A general practitioner can order all of them, and in most cases the results come back normal, confirming a dietary or microbiome-related cause.
Fecal Odor as a Diagnostic Frontier
It might seem trivial to worry about what your stool smells like, but researchers are increasingly interested in exactly this question. The volatile organic compounds released by stool are not random. They reflect the metabolic activity of your gut bacteria, the efficiency of your digestive organs, and even the presence of specific diseases. Electronic nose devices that use sensor arrays and pattern-recognition algorithms can analyze the volatile compounds in stool headspace and identify patterns associated with specific gastrointestinal illnesses.10PubMed Central. Diagnosing gastrointestinal illnesses using fecal headspace volatile organic compounds
This technology is still largely in the research phase, but the principle is sound and validated. Conditions like inflammatory bowel disease, Clostridioides difficile infection, and colorectal cancer each produce distinctive patterns of fecal volatile compounds. Your nose is a less precise instrument than an electronic sensor array, but it is picking up on real chemical signals. A persistent, unexplained change in stool odor is your nose’s crude version of a diagnostic readout, and while it’s usually nothing serious, it’s not unreasonable to pay attention to what it is telling you.