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Cat suddenly aggressive after brother returned home? What to do?

Updated On September 23rd, 2025

Pet's info: Cat | Mixed Breed | Male | neutered

I have two cats Colby and jack who are brother and get along great. They both went outside yesterday and Colby never came home. We found him today and he has a Scratch on his face. Ever since being home Jack has been hissing at him and trying to attack him. Why is jack all of a sudden angry and trying to hurt Colby? They are both around 3 years old and have never had problems

2 Answers

Most Helpful Answer

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Answered By Dr. Melanie, BVSc MS

Veterinarian

Published on April 3rd, 2020

Because Colby disappeared for a bit, he probably went on an adventure and came back smelling differently. Jack views him as a stranger, and that's why he's acting aggressively. I would separate the cats, and slowly reintroduce them to each other. Take Jack, and put him in his own room with his own food, water, and litter box. Keep the door closed, and for a few days, feed them both treats on other side of door so they associate each other's smell with a good experience. Then let Colby in Jack's room while Jack is in a cat carrier. They can smell each other, but not be in contact. If either becomes upset or show signs of aggression, remove Colby from the room, and try again later. Praise them when they interact appropriately. When they are interacting well with each other through the carrier, you can let Jack out of his room, and see how he interacts with Colby. Praise them when they interact appropriately, and separate them with any sign of aggression or upset. I would get a cat tree with lots of shelves to give the cats vertical space to get away from each other. I would keep Jack in his room when you go out until they are comfortable with each other.

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1Pet Parents found this answer helpful

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  • Image profile

    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on February 25th, 2019

    I would separate the cats, and slowly reintroduce them to each other. Take the cat who is aggressive, and put her in her own room with her own food, water, and litter box. Keep the door closed, and for a few days, feed them both treats on other side of door so they associate each other's smell with a good experience. Then let the other cat in the aggressive cat's room while the aggressive cat is in a cat carrier. They can smell each other, but not be in contact. If either becomes upset or show signs of aggression, remove the second cat from the room, and try again later. Praise them when they interact appropriately. When they are interacting well with each other through the carrier, you can let the aggressive cat out of her room, and see how she interacts with the other cat. Praise them when they interact appropriately, and separate them with any sign of aggression or upset. I would get a cat tree with lots of shelves to give the cats vertical space to get away from each other. I would keep the cat in her room when you go out until they are comfortable with each other. I hope this helps.

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