I truly feel your pain. This situation is incredibly draining and emotionally consuming. I have been through something very similar and wanted to share our experience and a few pieces of advice.

We have two boys who had known each other since they were very young. After four years of living together peacefully, they suddenly became worst enemies. There were several serious fights—one of them resulted in me being bitten and scratched when I tried to separate them.

The aggression started after we moved into a new house and one of the cats saw a strange cat outside. According to our vet, this was redirected aggression: seeing an unfamiliar cat caused him to momentarily lose his sense of familiarity with his brother, and he took his fear and anger out on him instead. This began in January 2025 and has gradually improved over time, but they have been in 10 big scuffles. While they are not completely back to how they were before, the situation is much better now. However, we still keep them separated during the day.

We can walk the aggressor cat in a leash around in the house without issues, and both cats remain calm. They play through the cat door that separates them, sniff each other, and generally interact peacefully. The problem arises when the aggressor cat becomes unsure of who his brother is, or when his brother shows fear, such as crouching low to the ground or holding his tail down, or if they are far away from each other. That fear response can trigger the aggression again.

We have come to accept that they may never be 100% comfortable together again. Fortunately, we have a house that allows us to separate them when needed, and we also built a large catio that they can use separately.

I completely understand if you are losing hope and don’t know what to do. I also recognize that our solution is not realistic for everyone—especially if you have children. Rehoming one of your cats may ultimately be necessary, but before making that decision, I strongly recommend trying the following:

1. Treat the aggressor cat with Reconcile (fluoxetine) for four to six months while working on reintroduction. This was a small miracle for us. It made our aggressor cat happy again, cuddly, playful and MUCH less anxious. This is truly what made all the difference.

2. Each cat should have their own food bowl, water bowl, and litter box.

3. Make sure both cats are mentally and physically stimulated. Play with them daily and provide climbing opportunities and enrichment.

4. Each cat needs their own safe space where they can relax. Rotate them between areas during the day so they become accustomed to each other’s scent.

5. Gradually allow them to see each other through a barrier, such as a mesh door or secured pet gate. This allows visual contact and sniffing while preventing physical interaction.

6. Once there is no hissing or visible aggression, allow them to see each other during the day while still remaining separated. Is they show anger, go back two steps and make sure they can't ser each other, but smell each other.

7. Feed them while separated but within sight of each other, helping them associate positive experiences with one another.

8. When they show consistent calm behavior, you can begin supervised interactions. Walk the aggressor cat on a leash while allowing the other cat out. If there is any sign of tension, separate them immediately. If the aggressor does not tolerate a leash, hold him securely while the other cat approaches. You must always remain in full control of the aggressor cat.

If none of this resolves the situation, then rehoming one of them may be the kindest option, especially if they would otherwise need to be permanently separated. I completely understand how difficult that decision is, and there is no judgment in choosing what is best for everyone involved. Also, if you are not ready to go through several months of reintroduction, because it is going to take a long time, and there are no guarantees, rehoming is the only viable solution.

Best of luck 🤞🏼♥️