Answer-first
My cats that got along are suddenly fighting — why, and what do I do?
First, rule out pain or illness in either cat with a vet — a cat that hurts lashes out, and sudden conflict between bonded cats is often medical or a trigger like redirected aggression. Then separate, de-escalate, and reintroduce gradually rather than 'letting them work it out.'Professional guidance
Any sudden behavior change gets a medical rule-out first (ASPCA). This page organizes the response; it does not diagnose.
Applies to: Homes where previously-friendly cats began fighting. A behavior + safety page with a medical-first gate.
Steps
Rule out the invisible causeFact
- 'Any aggression change should prompt a medical rule-out' — a cat in pain (dental, arthritis, illness) redirects onto a housemate
- A recent vet visit, a scary noise, or an outdoor cat at the window can trigger REDIRECTED aggression onto the nearest cat
- Book a vet check for the aggressor AND, if unclear, the victim
De-escalate safelyFact
- Never break up a fight with your hands — use a barrier or noise
- Separate the cats fully (different rooms) to reset — stress fuels more fighting
- Restore calm and routine before any face-to-face contact
Reintroduce like strangersProfessional guidance
- Run the full scent-first, gradual reintroduction protocol (as for new cats)
- Duplicate and separate resources: food, water, litter (n+1), resting spots
- Go at the more nervous cat's pace; setbacks mean slow down, not push
Normal vs keep-watching
Normal: Occasional hissing or a swat during play or minor squabbles is normal cat communication; sustained fighting, injuries, or hiding is not.
Keep watching: What preceded the change (vet trip, outside cat, new object), which cat starts it, and any injuries.
When to contact a vet. See a veterinarian promptly for a medical rule-out in either cat, and urgently for any bite wound (cat bites abscess fast). This page never diagnoses the cause of the aggression; a feline behavior professional helps if health is cleared and conflict persists.
What changes this answer
- Sudden vs gradual onset (sudden = medical-first)
- A recent trigger event (redirected aggression)
- Injuries present (urgent vet)
Common mistakes
- 'Letting them fight it out'
- Skipping the vet check
- Punishing the aggressor (raises stress, worsens it)
- Rushing reintroduction
Sources
- ASPCA (T2/T4 behavior) + AAFP/ISFM Environmental Needs Guidelines (T1, PMC11383066) · Tier 2 · verified 2026-07-17 · commercial bias noted · source conflict noted
- AAFP / American Association of Feline Practitioners (T1) — accompanies 2024 AAFP Intercat Tension Guidelines (JFMS) · Tier 2 · verified 2026-07-17 · commercial bias noted · source conflict noted
- Ohio State University Indoor Pet Initiative (T2 academic-veterinary); corroborated by AAFP (catvets intercat guide) and AAHA/AAFP Life Stage Guidelines · Tier 2 · verified 2026-07-17 · commercial bias noted · source conflict noted
Real questions cat owners ask
- Why did my cats suddenly start fighting?
- What is redirected aggression in cats?
- How do I reintroduce cats after a fight?
Note the trigger and date in This Week — patterns (dusk? after the window?) point to the fix. Set up your Cat Passport.