The cause of nonrecognition aggression is not entirely clear, but if a cat threatens his onetime friend after being separated from him for a time, he may be confused by the other cat’s smell. When dealing with cats who harbor these aggressive impulses, it’s always best to reintroduce the returning partner slowly, as if they’d never met.


Nonrecognition aggression occurs when one cat is uncharacteristically aggressive toward a companion cat after a period of separation. For example, after one cat returns home from a veterinary visit, the cat that stayed home is aggressive toward the returning cat, who may flee, freeze (hold still), or fight back.

The cause of this type of aggression is not fully understood. Possible causes include:

  • The returning cat is unrecognizable and/or perceived as a threat because he or she smells alien.
  • The returning cat is ill or behaving abnormally (for example, after an anesthetic procedure), and the aggressive cat perceives this as a threat.
  • The smell of alcohol or disinfectant on the returning cat reminds the aggressive cat of a negative experience.
  • The returning cat may have discharged his or her anal sacs at the veterinarian’s office or is otherwise emitting pheromones associated with stress. This may signal danger to the other cat, thereby causing him to become aggressive.

Symptoms and Identification

Aggression generally refers to threats or attacks, but that’s not always easy to see when it comes to intercat issues in general. Aggressive acts among cats can include hissing, growling, swatting, chasing, and biting — or it can come down to subtle incidents of intimidation unrecognized by owners as aggression.

Here are some rules for the two kinds of cats in a nonrecognition scenario.

Signs of an Attacking Cat (Left Behind):

  • Staring with constricted (small) pupils
  • Raised hair along the shoulders and tail
  • Facing the returning cat and appearing ready to pounce

Signs of a Defensive Cat (Returning to the Household):

  • Dilated pupils: the center (iris) of the eyes opens so that the pupils become large
  • Ears pressed back against the head
  • Arched back
  • Raised hair along the shoulders and tail
  • Facing the aggressor sideways
  • Hissing, spitting, and/or growling
  • Rolling onto back to fight if there’s no escape

Affected Breeds

All breeds of cats seem equally disposed to this kind of intercat behavior.


Here are some recommendations if you are faced with this problem:

  1. Never let cats fight it out. Cats don’t resolve their issues through fighting.
  2. Interrupting aggression or fighting by clapping hands loudly, squirting the aggressive cat with a water gun, or directing compressed air (without noise) at the aggressive cat can be more fruitful than allowing the cats to engage one another directly.
  3. Herding the aggressor into a separate room so that he can calm down (which might take several hours) can help diffuse the situation. If necessary, covering the aggressor with a large towel to help calm him while you are handling or moving him can reduce your risk of injury.
  4.  Don’t try to soothe the cats right away; give them time to calm down. An agitated cat may become aggressive toward any pet or person who gets close. If necessary, keep the aggressor confined overnight with food, water, and a litterbox.

It’s recommended that owners reintroduce two cats only when the aggressor has completely calmed down and is back to normal. It may help to reintroduce cats gradually through a screen, gate, or cracked door before allowing them full access to each other. Place cats’ food bowls on opposite sides of the barrier to encourage them to be close together while doing something they enjoy. Once both cats appear relaxed, open the barrier between them little by little. If the cats remain relaxed, they may be ready to be together again. If they show signs of aggression (e.g., growling, spitting, hissing, swatting), separate them again and restart the process of gradual reintroduction.

Two cats are likely to reestablish a relationship or at least tolerate each other, but future episodes of nonrecognition aggression are likely.

Any change in a cat’s behavior could also be a sign of a medical condition. If either cat’s behavior in such situations does not improve, further veterinary care is warranted. Contacting a certified applied animal behaviorist (CAAB or ACAAB after the last name) or a board-certified veterinary behaviorist (DACVB after the last name) is recommended if no biological cause of illness is identified.


To help prevent nonrecognition aggression, take the following measures after bringing a cat home from your veterinarian’s office and before reintroducing him or her to your other cat(s).

  • Ensure that a cat has fully recovered from sedation or anesthesia. If possible, bathe a cat to remove “veterinary” odors.
  • Keep returning cats separate from the other cat(s) to give them time to remember each other’s sounds and odors.

The following general guidelines can also help reduce aggression.

  • Neuter all cats. Unneutered males are especially prone to aggression toward each other. Separate the cats’ resources. Reduce competition between them by providing multiple, identical food bowls, beds, and litterboxes in different areas of the house.
  • Provide additional feline-friendly areas (e.g., hiding spots) to allow cats to space themselves out as they prefer.
  • Reward cats for getting along. Praise them or give them treats when they interact in a friendly manner.
  • Pheromones might help. Feliway (from Ceva Animal Health in St. Louis) is a product that mimics feline pheromones that may reduce tension between cats.

This article has been reviewed by a Veterinarian.