Aggression between cats in the same household is the most common form of feline aggression. This aggression can occur after a new cat is introduced into the house or between two (or more) cats that have lived together peacefully for many years. A frightened or startled cat will often attack another cat for no apparent reason.

A new odor or a change of appearance can elicit an aggressive response in a cat. The odor of an unfamiliar cat on a piece of clothing can trigger a violent reaction. As a result of this unfamiliar odor, the startled cat may attack a friendly housemate. When a cat see another cat (on the other side of a window) but cannot reach it, a friendly housemate is often the victim of an attack. If a longhaired cat receives a haircut, he (or she) may look unfamiliar to the other cats in the household. This unfamiliar looking cat may also become the victim of an attack.

The best response to this situation is to separate the cats. This should be done as quickly as possible. The two cats should be placed in separate rooms. A confinement period of one week is recommended. After the confinement period is over, both cats should gradually be reintroduced. If any aggression is noticed, the aggressive cat (or both cats) should be confined again.

Anti-anxiety medication is often useful for solving aggression problems between cats in the same household.

The medication is given during the period of confinement or when the cats are reintroduced. This medication should only be given under the supervision of a veterinarian.