My Cat Is Suddenly Attacking My Other Cat: What's Really Going On
- Lucia Fernandes
- Nov 8, 2024
- 14 min read
Updated: 6 days ago
Quick Answer
Sudden inter-cat aggression in a household where cats have previously coexisted is almost always a sign of internal arousal, not a change in how one cat feels about the other. The most common causes are redirected aggression triggered by an external event, a medical issue causing pain or disorientation, or a breakdown in the social tolerance that was always more fragile than it appeared. The attacking cat is not "turning mean." Her nervous system is overwhelmed, and the other cat is the closest available target.
You're not imagining it. She was completely fine with him for months, and now she hunts him through the house. You've tried the diffusers, the calming collars, the treats at a distance. Nothing is sticking. And the more you read, the more generic the advice sounds, none of it quite fitting what you're actually seeing.
After 15 years working with multi-cat households, I can tell you that sudden aggression between cats who previously coexisted is one of the most misunderstood behaviour problems there is. People reach for "dominance" as an explanation, or assume the cats have simply decided they don't like each other. In almost every case I've worked with, something more specific is happening, and once you identify it, the path forward becomes much clearer. This post walks through the most likely causes, how to tell them apart, and what to do in the short term while you get to the root of it.
Rule Out a Medical Cause First
Before attributing sudden aggression to behaviour, rule out pain, neurological changes, hyperthyroidism, or cognitive dysfunction. A cat who is hurting or disoriented may attack housemates she would normally ignore. If the aggression appeared very abruptly, if she is also acting strangely in other ways, or if she is over 8 years old, a vet check is the first step, not the last.
Why Is My Cat Suddenly Attacking My Other Cat?
1 - Redirected Aggression: The Most Common Cause Nobody Talks About
This is the scenario I see most often, and the one most frequently missed. Your cat was aroused by something she cannot reach: a cat outside the window, a sound, a smell carried in through a vent, a neighbour's dog. She has nowhere to direct that physiological activation, and the closest living creature, your other cat, becomes the target. It can look completely unprovoked because the original trigger has already disappeared by the time the attack happens.
The "seeking him out" behaviour you might be noticing is characteristic of this state. A cat in a prolonged state of arousal does not simply calm down once the trigger is gone. The nervous system stays activated, sometimes for hours, and the cat essentially looks for an outlet. This is not aggression born of dislike. It is aggression born of an overwhelmed stress response.
Redirected Aggression
A form of feline aggression that occurs when a cat is aroused or threatened by a stimulus it cannot directly access, and instead directs that aggression toward a nearby individual, human or animal. The target is not the cause of the aggression. The behaviour is driven by the cat's inability to complete the intended response to the original trigger.
Research
Redirected aggression is documented in veterinary behaviour literature as one of the primary causes of sudden inter-cat conflict in previously stable households. The arousal state can persist well beyond the removal of the initial trigger, which is why owners often cannot identify a cause. Recognising the time gap between trigger and attack is essential to diagnosis.
Beaver, B.V. (2003). Feline Behavior: A Guide for Veterinarians (2nd ed.). Saunders.
What to Do
Block visual access to the trigger: window film, rearranged furniture, or temporarily closing off the room where she watches outside.
Do not intervene physically during or immediately after an episode. A cat in redirected arousal can attack the person reaching in. Use a towel or blanket to block contact, or separate rooms with a closed door.
Give her a minimum of 24 to 48 hours of full separation from the other cat after any serious attack. Arousal takes longer to resolve than it appears.
Log the timing of attacks. If they cluster around certain times of day or rooms, you are looking for the external trigger.
2 - Fragile Social Tolerance: When They Were "Fine" But Not Actually Friends
Cats are not obligate social animals. Unlike dogs, they did not evolve to live in bonded groups with structured hierarchies. When multiple cats share a home successfully, they are often engaged in a process of careful avoidance and spatial negotiation, not genuine social bonding. This arrangement can look perfectly peaceful for months or years, and then collapse when something shifts the balance.
The shift can be surprisingly small: a change in the daily routine, a new smell on a person who came home, a rearranged piece of furniture that eliminates a preferred escape route, a period of increased outside cat activity, a new baby, a move. If your cats were always in the same space but rarely chose to be near each other, they may have been tolerating rather than accepting. The introduction of any additional stressor can tip that tolerance into open conflict.
In a six-cat household with one male, the social dynamics are particularly worth examining. Females in multi-cat environments often have strong spatial preferences and can experience social stress even when the household looks calm to human observers. The fact that she seeks him out specifically suggests he may have entered a space or used a resource she considers her own, and that claim is now being defended actively rather than through the subtle displacement she used before.
Coalition Dynamics
In feline social groups, individual cats form preferential relationships and spatial alliances that are not always visible to their owners. When these informal coalitions shift, or when an individual perceives a resource or spatial claim to be threatened, aggression can emerge suddenly between cats who previously appeared stable. This is not a personality change. It is the surface expression of pressure that had been building below a manageable threshold.
What to Do
Audit your resources: litter boxes, feeding stations, high resting spots, entry and exit points. In a six-cat home, the standard formula is one per cat plus one. If any of these are clustered, redistribute them.
Identify her core territory zones and ensure the male has no reason to pass through or near them. Physical separation of movement corridors is often more effective than calming products.
Do not attempt to force proximity. Feeding them near each other only works when baseline arousal is already low.
Consider whether any recent change in the household preceded the onset. The trigger is often two to three weeks before the first incident.
3 - Pain, Illness, or a Medical Change in Either Cat
This one works in two directions. The attacker may be in pain and therefore have a much lower threshold for arousal and aggression. Dental disease, arthritis, hyperthyroidism, and urinary discomfort are all conditions that can significantly shorten a cat's fuse without any other visible symptoms. A cat who was previously tolerant becomes reactive because every interaction carries a background of low-level pain.
The direction most people miss: the target cat may have changed. Cats communicate a great deal through scent and micro-signals that we cannot detect. If the male cat recently returned from a vet visit, was anaesthetised for a procedure, or has a developing medical condition, he may be emitting different olfactory information. To the female, he may smell and move like a different, unknown cat. The attack is not irrational from her perspective. She is responding to information she is genuinely receiving.
Research
Non-recognition aggression following veterinary visits is well documented in feline behaviour literature. The returning cat carries clinic scents and may behave differently post-procedure, triggering an attack from housemates who knew the cat before. Full scent reintegration, rather than immediate reunion, is the recommended management approach.
Overall, K.L. (2013). Manual of Clinical Behavioral Medicine for Dogs and Cats. Elsevier Mosby.
What to Do
Schedule a vet check for the attacking cat specifically. Ask your vet to assess for pain, thyroid function, and any neurological changes, particularly if she is over 7.
If the aggression began within a week of either cat's vet visit, manage it as non-recognition aggression: full separation, then a slow scent reintroduction before any visual contact.
Rub a cloth on one cat and place it in the other cat's space before attempting any shared space. Repeat for several days before progressing.
4 - Accumulated Stress: When the Bucket Overflows
Cats have what behaviorists sometimes describe as a stress bucket. It fills gradually, from sources that each look manageable in isolation: seasonal shifts in outside cat activity, a neighbour renovating nearby, a child home more than usual, a change in your own schedule. None of these would cause a problem on their own. Together, they push the cat over a threshold, and suddenly behaviour that was previously contained is not.
This explains why the standard advice, diffusers, calming supplements, more resources, is not working for you. Those tools work best as prevention, when the bucket is half full. Once it has overflowed, you need to actively reduce the number of incoming stressors, not just add calming agents into a system that is already saturated.
A household move, which you mentioned is coming, is one of the highest-stress events in a cat's life. It is worth considering whether her system is already partly braced for disruption she can sense is coming, through changes in your own behaviour and the household's atmosphere, and that this is lowering her threshold now, months ahead of the actual event.
What to Do
List every change in the last three months, including things that seem unrelated. Look for the accumulation, not a single cause.
Reduce active stressors before adding calming interventions. Block the outside cat view, adjust schedules where you can, restore predictability.
Create at least one space in the home where the attacking cat is never approached, by you, by the other cats, by visitors. She needs a place to regulate.
If the move is coming within six months, begin scent and spatial preparation now. The calmer the baseline before the move, the lower the risk of serious escalation after.
From My Practice - Real Case
Maya and Pip: When "Sweet" Turned Sudden
Maya was a four-year-old spayed female who had lived with Pip, a younger male, for eight months without incident. Her owner contacted me after Maya began stalking and attacking Pip several times a day, appearing to come out of nowhere. She described Maya as "obsessed" with finding him.
When I reviewed the household timeline, we found that a stray cat had begun visiting the garden about four weeks before the first attack. Maya, who favoured the back windows, had had multiple prolonged arousal episodes watching this intruder. The attacks on Pip followed a consistent pattern: they intensified on days when the stray appeared. Once the owner used window film to block Maya's sightline to the garden and gave her three weeks of managed separation from Pip with a carefully structured reintroduction, the household stabilised. Maya and Pip are now back to sharing resting spaces, and the stray cat has been trapped and neutered through a local TNR programme.
What to Do Right Now: Managing the Immediate Situation
What to do right now if your cat suddenly attacking other cat.
Before anything else, stop trying to force coexistence. Every unsuccessful interaction is adding arousal to a system that already has too much. The goal right now is to stop the conflict from deepening, not to rush toward resolution.
1. Separate completely, and mean it
Full separation means they cannot see, hear, or smell each other in a way that creates arousal. Baby gates are not enough if she can hear him and become frustrated. Each cat needs her own litter box, water, food, and resting spots in her zone. This is not a punishment. It is a reset.
2. Vet check for the attacking cat
Ask specifically about pain screening and thyroid levels. Mention that the aggression was sudden and unprovoked in presentation. If your vet does not take behavioural changes seriously as a medical symptom, it is worth a second opinion from a feline-specialist vet.
3. Log everything for two weeks
Time of day, room, what happened in the hour before, what was happening outside, who was home. Patterns that are invisible in the moment become obvious in a log. Bring this to any behaviour consultation or vet visit.
4. Begin scent exchange before any visual contact
Swap bedding between the two zones. Feed each cat near the separation barrier, but not where they can see each other, working toward the barrier over several days. Only add visual contact once there are two full days without any threat displays at the barrier.
5. Plan the reintroduction as if they are strangers
The formal cat-to-cat reintroduction protocol works even for cats who previously lived together, because the relationship has effectively been broken. A slow reintroduction over two to four weeks, with careful management of each stage, gives you a far higher success rate than simply hoping the aggression passes. I have a full guide on this process here.
Is This Redirected Aggression or a Relationship Breakdown?
Not all sudden inter-cat aggression comes from the same place, and the management is different depending on which pattern you are dealing with. Redirected aggression resolves relatively quickly once the trigger is identified and removed. A relationship breakdown takes longer and requires a structured reintroduction, even between cats who previously lived together.
The clearest way to tell them apart is to look at what happens between attacks. A cat who is calm and relaxed when the trigger is absent is showing you redirected aggression. A cat who remains tense, watchful, and avoidant even in quiet moments is showing you something that has shifted at a deeper level.
Use the table below to identify which pattern fits what you are seeing.
Work through these steps in sequence. Each one matters.
Before anything else, stop trying to force coexistence between your cats. Every unsuccessful interaction is adding arousal to a system that already has too much. The goal right now is to stop the conflict from deepening, not to rush toward resolution.
Work through these steps in sequence. Each one matters.
If your cat is directing arousal outward through aggression, the missing piece is usually not a calming product. It is a structured outlet. The Advanced Play Handbook covers inter-cat dynamics, arousal thresholds, and how to use play as a management and reintroduction tool in multi-cat households.
Frequently Asked Questions
Can cats who start fighting suddenly go back to normal?
Yes, in most cases, with the right management. The prognosis is best when the aggression is caught early, when the underlying trigger is identified and removed, and when reintroduction is done slowly and systematically rather than by simply hoping the cats work it out. Cats who have had a redirected aggression episode and are correctly separated and reintroduced often return to their previous level of tolerance or better. A true relationship breakdown requires more work, but it is not a permanent state in the majority of cases.
My cat seeks out the other cat to attack him even when I try to distract her. Is that normal?
It is a recognisable pattern, and it is one of the clearest signs that the arousal driving the aggression has not resolved. A cat in a prolonged arousal state is essentially scanning for the outlet she has already identified, and distraction alone rarely interrupts that drive. This is why complete separation is necessary, not just supervision. The seeking behaviour typically fades over 24 to 72 hours of genuine separation, though some cats need longer. If it is still intense after a week of separation, a vet check is warranted to rule out a medical or neurological component.
We have tried calming products and nothing is working. What else can we do?
Synthetic pheromone diffusers and calming supplements work on the ambient anxiety level in a household. They are most effective as background support when the primary stressor has already been identified and reduced. If the underlying trigger is still present, no calming product will override the arousal response it is creating. The next step is not a stronger calming product. It is identifying and removing the stressor. If you cannot do that without help, a consultation with a certified feline behaviourist is the most efficient use of your time. Some owners find natural options such as valerian-based sprays or species-appropriate environmental adjustments more aligned with their approach, and these can support a calmer baseline alongside stressor reduction.
We are moving to a smaller home in a few months. Should I wait until after the move to address this?
Please do not wait. Managing an active inter-cat conflict through a house move, in a smaller space with all the disruption and arousal that comes with it, is extremely difficult. Stabilise the household as much as possible before the move, then manage the move itself as a planned reintroduction into the new space, keeping the cats separated initially even if they were getting along again, and reintroducing the shared space gradually.
Could my female cat be reacting to the fact that he is male, even though he is neutered?
In a neutered male who has been in the household for over a year, sex-based social conflict is unlikely to be the primary driver. What is more relevant is the social structure of your specific household. With six cats, five of whom are female, the group dynamic has a particular shape, and the addition of a younger male may have created spatial or social pressures that took time to manifest. This is a background factor, not a cause in itself, and it does not change the management approach: identify the stressor, separate safely, and reintroduce carefully.
I have read everything and I genuinely do not know what is causing this. What should I do?
Two weeks of detailed behaviour logging is the first step. In many cases the pattern only becomes visible when you write it down: time of day, room, what happened in the hour before, what was happening outside. If logging does not reveal a trigger and the standard management steps are not producing any reduction in the behaviour, a direct consultation with a certified feline behaviourist is the most efficient next step. For complex multi-cat cases it is a much faster route than months of trial and error.
Key Takeaways
Sudden inter-cat aggression is almost always a sign of internal arousal in the attacking cat, not a change in how she feels about the other cat. The most common cause is redirected aggression, an external stimulus she cannot reach triggers a state that then discharges onto the nearest available target.
Before assuming the problem is behavioural, rule out pain and medical changes in both cats, particularly if the onset was very abrupt or if either cat had a recent vet visit. A cat in pain has a much lower threshold, and a cat returning from the vet may smell and behave like a stranger.
Calming products and proximity feeding work best as prevention. Once the threshold has been crossed, active stressor reduction is more effective than adding calming aids into a system that is already saturated.
Complete separation followed by a formal scent-based reintroduction protocol is the most reliable path to restabilising the household. In a home with six or more cats, auditing resources (litter boxes, feeding stations, high spots, and escape routes) is an essential early step and often reveals structural problems that no calming intervention can fix.
If a house move is coming, do not wait for the crisis. Arousal thresholds can drop weeks before the event itself, as cats pick up on changes in the household atmosphere. Begin preparation early.
Final Thought
The hardest part of inter-cat aggression is not the conflict itself. It is watching two cats who used to sleep near each other become strangers, and not knowing whether they will ever find their way back.
Most of the time, they do. Not always to the same closeness, but to a workable, stable arrangement where no one is hunting and no one is hiding. That is a realistic goal, and it is achievable in the majority of cases when the underlying cause is identified and the reintroduction is handled carefully.
What I want you to take from this: the attacking cat is not broken, and she is not mean. She is overwhelmed. Her nervous system fired in response to something (a trigger, a pain, a shift in the household that tipped a fragile balance) and she has not yet had the conditions to come back down. Separation is not giving up on her. It is giving her system the reset it needs.
The cats in the cases I work with most often do not fail because the problem was too complex. They fail because the pressure to reunite quickly overrides the patience the process requires. Give it time. Give it structure. The relationship is not over. It is just paused.
Free Guide
The Pair-to-Pair Reset Method: Reintroducing Cats Who Have Had a Conflict
A step-by-step reintroduction protocol for cats who previously lived together but can no longer share space safely.
Continue Exploring
Aggression in Cats: complete overview of feline aggression types and triggers
Fear and Anxiety in Cats: understanding the stress response at the root of most behaviour problems
Anxiety in Cats: signs, causes, and what to do when a cat is chronically overwhelmed
Separation Anxiety in Cats: when the problem is attachment, not aggression
How to Calm a Stressed Cat: practical steps for reducing arousal in the short and long term
Environmental Enrichment: how space, resources, and territory affect feline behaviour
References
Beaver, B.V. (2003). Feline Behavior: A Guide for Veterinarians (2nd ed.). Saunders.
Overall, K.L. (2013). Manual of Clinical Behavioral Medicine for Dogs and Cats. Elsevier Mosby.
Bradshaw, J.W.S., Casey, R.A., & Brown, S.L. (2012). The Behaviour of the Domestic Cat (2nd ed.). CABI.
Amat, M., Manteca, X., Brech, S.L., & Fatjó, J. (2008). Evaluation of inciting causes, alternative targets, and risk factors associated with redirected aggression in cats. Journal of the American Veterinary Medical Association, 233(4), 586–591.
Stella, J.L., & Buffington, C.A.T. (2014). Individual and environmental effects on health and welfare. In The Domestic Cat: The Biology of its Behaviour (3rd ed.). Cambridge University Press.
I've been losing my mind over this for the past month. My 4-year-old female was absolutely fine with my younger male for almost a year, and then one day she just… snapped. Now she hunts him through the house. I have Feliway everywhere, I've tried the calming drops, I've rearranged the furniture. Nothing. She seems completely normal with my other cats, it's only him. What kills me is that she looks so deliberate about it. She's not reacting, she's searching. I've been doing 12-hour separations but the minute they're back together it starts again. I don't know if I'm missing something obvious or if this is just… who she is now.