Why does my cat’s poop smell so bad?

The calls I fielded at the clinic intake desk about litter smell came in two flavors. The first was the resigned one: “the box always smells a bit, that’s just cats, right?” The second was the worried one, and it’s the one worth listening to: “the smell changed, it got sharp and awful almost overnight.” A cat’s stool is never going to smell like nothing. But a sudden jump in how bad it smells is real information, not just an unpleasant fact of apartment life. It usually points at a short list of causes you can work through, starting with the bowl and ending, if it doesn’t settle, at the vet.

The short version

• Some odor is normal. A sudden change in how foul it smells is the part worth acting on.
• Diet is the first suspect. An abrupt food switch, or a food your cat digests poorly, can turn stool foul within days.¹
• Change foods slowly, over about a week, by mixing the new food into the old.³
• Loose, greasy or oddly coloured stool that smells terrible often means food is moving through too fast to be absorbed.¹
• Parasites like Giardia and coccidia are a classic cause, especially in kittens, newly adopted cats and multi-cat homes.²
• Black tarry stool, blood, mucus, weight loss or a smell that won’t settle in a day or two means call the vet.¹²

A little smell is normal. A lot is new information

Healthy cat stool has an odor, and it always will. The gut is full of bacteria whose job is to break down what the small intestine didn’t absorb, and that fermentation produces smell as a matter of course. So the useful question isn’t “does it smell,” it’s “did it change.” A stool that has always been a bit unpleasant and is stable is very different from one that suddenly turns eye-watering. In a small apartment you notice that shift within a day, which is actually an advantage. The smell is a gauge on your cat’s digestion, and a sharp move on that gauge is worth a second look rather than a scented candle. For the wider picture on how an indoor cat’s gut is supposed to work, our indoor cat digestion guide lays out the baseline.

Start with the food

Diet is the first thing to look at, because it’s the most common cause and the easiest to fix. Cornell notes that a short bout of foul, loose stool is often self-resolving or readily remedied with a change of diet, and that an abrupt switch in food by itself can set off a few days of diarrhea while the gut adjusts.¹ The mechanism is straightforward: when a food is hard for your cat to digest, more undigested material reaches the colon, where gut bacteria ferment it and produce the gases that make stool smell. A richer or lower-quality food, a new protein or table scraps can all push things in that direction.

The fix is boring and it works. Pick one complete, good-quality food your cat does well on and stay with it, rather than rotating on a whim or topping the bowl with human food. When you do need to change foods, go slowly. A gradual switch over about a week, mixing an increasing share of the new food into the old, gives the digestive system time to adapt instead of forcing it to cope all at once.³ If a diet change lines up with the smell getting worse, that timing is your clue.

When it’s not the food: absorption and speed

If the diet is steady and the smell is still bad, the next thing to consider is how well and how fast your cat is absorbing that food. When something makes waste move through the intestines too quickly, the body can’t pull out the nutrients and water it should, so the stool comes out soft, watery, sometimes gray or yellow and uncharacteristically foul.¹ That undigested material is exactly what feeds the smell. In an adult cat, a change in defecation that isn’t clearly dietary is most often tied to the gastrointestinal tract itself, from an inflammatory condition like inflammatory bowel disease to an infection.¹

Sometimes the source sits outside the gut entirely. Recurrent or extended foul diarrhea can be a sign of an overactive thyroid, kidney or liver disease or other systemic problems, which is why a stool issue that drags on deserves a proper workup rather than another bag of food.¹ None of this is meant to alarm you. Most smelly-stool episodes are short and diet-related. But a persistently foul, loose stool is the body telling you the food isn’t being handled well, and that’s a message for your vet.

Parasites are a classic offender

If a foul stool shows up in a kitten, a newly adopted cat or one of several cats sharing a home, put parasites near the top of your list. Gastrointestinal parasites are common in cats, with prevalence as high as 45% in some populations, and they tend to cause loose, mucousy or bloody stool along with a dull coat and a poor appetite.² Giardia, a one-celled organism that lives in the small intestine, is a well-known cause of acute or chronic diarrhea and spreads readily in multi-cat households and among cats under a year old.² Coccidia do much the same in kittens, where they can damage the intestinal lining and cause mucousy diarrhea.²

One parasite sign is an outright emergency. Hookworms feed on blood, and a heavy infection can leave the stool black and tarry from digested blood, which means a cat losing enough blood to be in danger.² That is a same-day vet visit. For the rest, diagnosis needs a microscope, not a guess, and because parasites like Giardia don’t shed continuously your vet may want more than one stool sample.² Treatment is a vet-prescribed dewormer plus good hygiene, since reinfection is common and some of these parasites can pass to people. Daily litter scooping and a clean box are a real part of breaking the cycle.²

What you can actually do at home

A few things are squarely in your hands. Keep the diet consistent and complete, and make any change a slow one. Keep fresh water down and don’t leave wet food out long enough to spoil. Stay on whatever parasite-prevention and deworming schedule your vet has set, because that’s the single biggest lever against the parasite causes. Scoop the box at least once a day, which both lets you spot a change early and cuts the risk of a cat reinfecting itself. And when you do book a vet visit, bring a fresh stool sample, because it saves a step and gives the lab something to work with. If you want to rule out the other end of the digestive tract while you’re at it, our guide on telling a hairball from a vomit covers the upstream version of the same detective work.

When to call the vet

Here’s the line I’d draw. A single foul-smelling stool after a new food, with your cat otherwise bright and eating, is worth watching but rarely worth panicking over. Call the vet if the loose or foul stool lasts more than a day or two, or if it comes with a poor appetite, low energy or vomiting, because that combination points past a simple diet blip.¹ Blood or mucus in the stool, black tarry stool, ongoing weight loss or a smell that simply won’t settle all warrant a call too. One more thing worth saying plainly: don’t reach for human anti-diarrhea medicines, because several are toxic to cats and can make a manageable problem dangerous.¹ For the broader map of which symptoms mean wait, which mean call today and which mean go now, our roundup of feline health red flags puts this in context.