Cat Urine Smells Like Ammonia: Is It Her, or Is It the Box?

When cat urine smells like ammonia, the first thing that smell is reporting is time. Fresh urine carries almost no ammonia. Ammonia is what turns up once urine has been sitting, so a sharp hit when you walk past the tray is usually telling you something about the tray. What points at your cat is whatever arrives alongside the smell: more drinking, straining, blood, weight coming off, or a cat who has suddenly started going somewhere else.

Sorting those two apart is most of the work, and it is worth doing in that order. A urine sample settles in one afternoon what no amount of sniffing can, so this is background for the conversation with your vet rather than a substitute for it.

Before the detail, the shape of the decision:

• Same day. She is going to the tray over and over and producing little or nothing, crying, or straining hard. This one is not about smell at all, and in a male cat it is an emergency.
• This week. The smell changed and something else changed with it: she is drinking more, losing weight, going outside the tray, or there is blood or pink in the litter.
• Worth noting, start with the tray. The smell changed and she is otherwise herself, eating and drinking and moving as usual. Work through the four tray checks below first. Most of the time that is where the answer is.

Why cat urine smells like ammonia

Because something is eating it. Urine leaves the body as urea, not ammonia, and McGill University's Office for Science and Society describes what happens next: After a while, bacteria decompose the urea and give off an ammoniacal odour characteristic of stale old urine. The ammonia is a by-product of decomposition, which means the strength of the smell tracks how long the urine has been sitting and how warm and damp it has been while it sat.

Leave it longer and it gets worse in a different direction. McGill notes that The second stage of the decomposition process emits mercaptans, compounds which also give skunk spray its nasty smell. That second stage is the smell people describe as old, sour, or set-in, and it is the reason a spot on a carpet keeps announcing itself weeks later.

The practical consequence runs through everything below. If you catch the ammonia when you scoop, or when you lean over the tray, you are smelling chemistry that happened in the litter. If you catch it the instant she goes, before anything has had time to sit, that is a different observation and a better reason to look at her.

When the smell is the box, not the cat

Four things change how much a tray smells, and every one of them is yours to adjust. Ohio State's Indoor Pet Initiative, a resource written by their veterinary college, is unusually blunt about the baseline: Odor should not be a problem if the litter box is kept clean.

How often you scoop. Their standard is the litter box must be scooped daily and washed weekly with mild dish detergent. Daily scooping removes the urine before the bacteria have finished with it, which is the whole mechanism. An older cat who is producing more urine than she used to may need twice that.

Whether it has a lid. This is the one that surprises people, because a hood looks like it should contain a smell. Ohio State says the opposite: Remember that covered litter boxes will concentrate odors more than uncovered boxes, and that Your cat may refuse to use a covered box if it is not kept very clean. A hood traps the smell for her and releases it at you in a lump every time the flap moves. For an old cat there is a second reason to skip it: Low sided boxes are appropriate for small kittens and older cats that have trouble getting in and out of the litter box, and low-sided trays are mostly open ones. There is more on choosing between them in the litter box guide for senior cats.

What you last bought. A change of litter can change the smell on its own, without anything happening to your cat. Depth matters too: Ohio State suggests you Fill boxes with enough litter to permit the cat to scratch and bury waste (usually about 2 inches deep), because a thin layer soaks through to the plastic and the plastic holds the smell. If you do switch, do it slowly, since their advice once a cat is settled is don't change types or brands or the cat may refuse to use the litter box. Which litter suits an old cat is its own question, covered in the senior cat litter guide.

The weather. Humidity is worth a thought, and here I will be straight about what I know. Clumping litter draws moisture out of the air as readily as out of your cat, and owners in damp climates do report the tray turning sharp during a muggy stretch with nothing else changed. I have not found a veterinary source that measures this, so treat it as something to check rather than something established: if the smell arrived with a run of humid days, try scooping twice a day and changing the litter out more often before you go hunting for a cause inside your cat.

If the cat litter box smells like ammonia and your cat is eating, drinking, and behaving normally, those four are where I would spend the first week.

Does kidney disease make cat urine smell stronger?

Usually the reverse, and this one is worth getting straight because the intuition runs the wrong way. Failing kidneys lose the ability to pack waste into a small volume of water. Cornell's Feline Health Center describes the result: As the kidneys lose their ability to concentrate urine effectively, cats may begin to urinate greater volumes and drink more water to compensate. It is measurable early, too. Cornell notes that A low urine specific gravity, indicating poorly concentrated urine, can be one of the earliest indicators of CKD in the cat.

Dilute urine is weaker smelling urine, not stronger. So a cat whose pee has become sharper is not, by that sign, telling you her kidneys are failing.

What confuses the picture is that the tray still gets worse. More volume means the litter saturates faster, more urine sits in contact with more litter, and the room ends up smelling more even while each individual puddle smells less. Owners of cats with chronic kidney disease often describe exactly this: a tray that needs doing twice as often as it used to. The smell in the room went up. The smell of the urine went down. Both are the same fact about her kidneys, and only the second one is the diagnostic clue, which is why the sign that actually matters is drinking more and losing weight rather than anything you can smell.

What changes the smell from her side

Three things, and only one of them is an emergency.

Concentrated urine. The inverse of the kidney case. A cat who is drinking less, or losing water faster than she takes it in, produces a smaller volume of more concentrated urine, and concentration is what makes a smell sharp. The Merck Veterinary Manual's advice for kidney patients works as a general principle: Cats should have access to plenty of fresh, clean water to encourage water intake (so the cat will have less concentrated urine). If her water bowl has moved, or the fountain has been off, or the weather turned hot, that is worth checking before anything else. There are practical ways to get more water into a senior cat if she is a reluctant drinker.

Infection. Strong smelling urine in older cats deserves more weight than it does in a young one, because the odds shift with age. VCA notes that bacterial infections of the urinary tract are more common in cats over ten years of age. An infection will not announce itself by smell alone, but it travels with signs you can see: VCA describes affected cats often spend a long time straining in the litter box while passing only small quantities of urine, along with blood in the urine and urinating in unusual places. Any of those with a changed smell is a call this week, and the full picture is in the guide to feline lower urinary tract disease.

A blockage, which is not about smell at all. It belongs here only because it lives in the same tray and gets missed while people are thinking about odor. VCA singles out complete urinary tract obstruction, resulting in the inability to urinate (mostly in male cats) and says it is essential to seek immediate veterinary care because it can be a life-threatening complication if untreated. A cat, especially a male one, going to the tray again and again and producing nothing needs a vet today, whatever the tray smells like. A cat who has started lying down in the tray between those trips needs one urgently.

The instinct that makes it worse

The natural response to a smell is to cover it, and with cats that backfires twice over.

Ohio State is direct about scented products: Many cats are put off by the odor of scented or deodorant litters, and This is also a good reason not to place room deodorizers or air fresheners near the litter box. Their finding on what cats actually want is plainer still: most cats prefer fine-grained, unscented litters.

Then comes the part that turns a smell problem into a floor problem. Most cats will avoid using a dirty litter box in favor of a cleaner place, and Ohio State puts the test in your own hands: If you find the odor offensive, the cat will also find it offensive and may not want to eliminate there. A cat who stops using a tray she finds unpleasant goes on the rug instead, and now the smell is in the house rather than in a box you can empty. This is one of the ordinary reasons behind a senior cat peeing outside the litter box, and it is fixable in an afternoon.

So the order is: clean it, do not mask it. A masked tray is still a tray she is avoiding.

How to get rid of cat urine smell

Once urine is on a floor or a mattress, ordinary cleaning does not finish the job, because the urea that feeds the smell soaks in and keeps feeding it. Two rules do most of the work.

Speed. Ohio State's cleaning guidance opens with it: The key to success is to thoroughly clean the area, as soon as possible. Blot up everything you can before you put anything else down.

An enzymatic cleaner, not a household one. Their instruction for hard floors is Blot up as much liquid as you can, then use an enzymatic cleaner. The reason these work is chemical rather than cosmetic: McGill explains that the enzymes break up the urea into carbon dioxide and ammonia, two gases that evaporate quickly. The enzyme removes the fuel. A scented spray only sits on top of it, and your cat can still smell what is underneath.

One warning is worth its own line, because so many household cleaners fall foul of it. Ohio State says plainly: Never use ammonia or ammonia-based products on the carpet. To a cat, an ammonia cleaner smells like another cat has already gone there, which is an invitation to go there again. Check the label on whatever is under your sink before you reach for it.

When to see the vet

Book a visit this week if the smell changed alongside anything else: more drinking, more urinating, weight loss, blood in the litter, straining, or accidents outside the tray. Any of those makes a urine test worth doing, and a urinalysis answers in one go what the smell can only hint at. It is also a reasonable thing to raise at her routine senior check-up, where a urine sample is often part of the visit anyway.

Go today if she is going to the tray repeatedly and producing little or nothing, especially if she is male, or if she is crying, hunched, or off her food.

And if she is well in every other respect, the honest answer is that the tray is the likelier culprit, and a week of daily scooping will tell you more than worrying will.

Frequently asked questions

Why does my cat's pee smell so strong all of a sudden? Something changed, and it is more often the tray than the cat: a new litter, a hood, a stretch of humid weather, or simply a few days of lighter scooping. Ammonia forms as bacteria break urine down over time, so anything that lets urine sit longer makes the smell stronger. Look at your cat if the smell arrived with other changes, such as drinking more, straining, blood, or accidents.

The cat litter box smells like ammonia but my cat seems completely fine. Should I worry? Probably not yet, and there is plenty to try. Scoop daily, wash the tray weekly, take the lid off if it has one, and use enough litter to let her bury properly. Ohio State's position is that Odor should not be a problem if the litter box is kept clean. If the smell survives a week of that, then it is worth a vet visit.

Does kidney disease make cat urine smell stronger? Generally no. Kidney disease makes urine more dilute, and Cornell notes that A low urine specific gravity, indicating poorly concentrated urine, can be one of the earliest indicators of CKD in the cat. The tray may well smell worse because there is more volume in it, but the urine itself tends to smell less. The kidney signs to watch are thirst, urine volume, and weight.

My cat's breath smells like ammonia. Is that the same thing? No, and it is a more urgent observation. Ammonia-like breath in an older cat is a reason to have her kidneys checked, and it belongs to a different set of causes than a smelly tray. That one is covered in senior cat bad breath.

The smell started right after I switched litter brands. Is it the litter? Very possibly. A new litter can change how urine smells and how well it is absorbed, and it can also put your cat off the tray. Ohio State's advice is not to change types or brands once a cat is settled, and if you must, offer the new litter in a second tray beside the old one so she can choose.

Is an air freshener near the box a bad idea? Yes. This is also a good reason not to place room deodorizers or air fresheners near the litter box, as Ohio State puts it, because cats are put off by scent near where they go. Masking the smell risks losing the tray altogether.

Biscuit's tray needs doing twice a day now, where it used to be once, and for a while I read that as something getting worse. It is mostly arithmetic: her kidneys send more water out than they used to, the litter fills sooner, and the room tells me about the volume rather than about her. So I scoop twice, keep the tray open and low, and save my worrying for the things a nose cannot measure anyway, which are her weight and how much she drinks.

Sources

• Cornell Feline Health Center, Chronic Kidney Disease
• McGill University Office for Science and Society, Why does cat urine smell so bad and what can I do about it?
• Ohio State University Indoor Pet Initiative, Litter Boxes
• Ohio State University Indoor Pet Initiative, How to Clean-Up Cat Urine
• Merck Veterinary Manual, Noninfectious Diseases of the Urinary System of Cats
• VCA Animal Hospitals, Cystitis and Lower Urinary Tract Disease in Cats