Why cats that got along for years suddenly start fighting

Two cats that have shared a home peacefully for years and then, seemingly overnight, start hissing, blocking doorways, or avoiding each other. This is one of the more disorienting things a multi-cat household can go through, because nothing about the relationship looks like it changed. Usually something did. It’s just not obvious yet.

A vet visit is the first thing to rule out

The single most common cause of sudden conflict between previously friendly cats is that one of them came back from the vet smelling different. Cats identify each other largely by scent, and a trip to the clinic can temporarily mask or alter that scent enough that the other cat treats a familiar housemate like a stranger. This is called non-recognition aggression, and it usually resolves within a few days on its own, faster if you separate them briefly and reintroduce with scent-swapping the way you would new cats.

An outdoor cat at the window

If one of your cats spends time near windows or doors, a stray or neighborhood cat passing by outside can trigger redirected aggression: the stressed cat can’t reach the outdoor cat, so it turns on the nearest available target, which is often the other cat in the house. Check whether the timing of the fights lines up with anything visible from a specific window.

A health issue in one cat

Pain changes behavior before it’s obvious to you. A cat with early arthritis, a dental problem, or a UTI may become more irritable or less able to signal normal social cues, which the other cat picks up on and reacts to. If the conflict is new and you can’t identify an environmental trigger, a vet check for the cat that started the aggression is worth ruling out before assuming it’s purely behavioral.

A shift in the resource layout

Something as small as moving a litter box, changing feeding times, or a piece of furniture that used to give one cat an escape route can quietly tip a balance that had been stable for years. Cats often don’t need a resource shortage to feel resource-insecure. A change in the layout can be enough.

What to do while you figure it out

Separate them the way you would for a first-time introduction: different rooms, scent swapping, then slow supervised reintroduction. Resist the urge to force them back together to “get it over with.” That tends to add a second bad memory on top of whatever triggered the original conflict, making the whole process take longer.