Aggression in Cats After a Trip to the Veterinarian

Most pet professionals and pet owners know the common types of aggression in cats are petting-induced, predatory, fear-induced, play, redirected and territorial. But there is a little-known but commonly seen form of aggression by the name of non-recognition aggression.

What is non-recognition aggression?

Non-recognition aggression is a type of aggression that occurs between household cats who are separated and then reunited. One cat leaves the house for some time (such as a visit or a stay at the veterinary hospital) and when he returns, is met with uncharacteristic aggression by the resident cat(s). The returning cat acts defensively, not understanding what provoked the attack, which escalates the situation further. The term ‘non-recognition aggression‘ describes the situation to a tee, as the resident cat quite literally doesn’t recognise his feline sibling after a period apart. A trip to the veterinarian is the most common scenario, but any situation in which one cat leaves the house, such as a trip to the cat groomer or a cat show can provoke aggression.

This kind and level of aggression can take on many forms including hissing, growling and swiping to a full-blown attack on the returning cat.

What causes non-recognition aggression?

For non-recognition aggression to occur, there must be two or more players.

  • The returning cat – Who is attacked
  • The resident cat – The aggressor

The cause:

Professionals still don’t completely understand the cause of non-recognition aggression in cats, but speculate it may be due to the following:

The returning cat smells different

Cats rely heavily on their sense of smell, much more than humans who are visual. A cat who has returned from the veterinarian will have a different smell and body chemistry which causes the resident cat to perceive the returning cat as a stranger. The returning cat can take on the smell of the practice itself, as well as medications or treatments (such as shampoos).

The new smell may remind the resident cat of a previously stressful situation himself

The ‘vet smell’ on the returning cat triggers in the aggressor its own negative experiences of a trip to the veterinarian. Let’s face it; most cats aren’t keen on the vet.

Change in behaviour of the returning cat

The behaviour of the returning cat is unusual due to stress, medications, pain, sickness or anesthesia wearing off.

Change in appearance of the returning cat

He may have had some of his fur clipped, is wearing a bandage, an Elizabethan collar of has had a part of his body amputated.

What can you do?

Firstly, know that this does happen quite often, and it usually resolves itself in time.

When you pick up a cat from the vet, if possible, bring along a piece of bedding (such as a blanket used to line a cat bed) that has the scent of the resident cat and/the house. Place this in the cat carrier for the cat to sit on. It will help to transfer familiar household scents on to the returning cat.

Spray the carrier with Feliway which is a synthetic pheromone that mimics the cat’s own facial pheromones which are calming. This can help to reduce anxiety in the returning cat on his journey home.

Use a Feliway plugin in the home too, to help calm the resident cat during this period.

If the returning cat is otherwise well:

Bring the cat inside in his cat carrier and leave him there for the resident cat to sniff and check out for a few minutes. Watch how the resident cat reacts to the returning cat.

If both cats appear to be fine with each other, open the cat carrier and let the returning cat out. Monitor closely and separate (see below) if any signs of aggression develop.

If the resident cat hisses or growls at the returning cat in his carrier, move the carrier to another room and set up a temporary home for the returning cat. Provide food and water bowls as well as a litter tray. Use a towel to wipe over the resident cat and then transfer the scent onto the returning cat. Leave him in the room for 12-24 hours; this will give him time to groom and re-acquire his normal scent as well as the scent of the house.

Re-introduce after 24 hours and watch for signs of aggression. It can help to open the door and feed both cats on either side of the door during this period of re-introduction.

If the returning cat is still sick, or groggy from an anesthetic:

Ideally, the cat should be brought home once the effects of the anesthetic have worn off, but sometimes they will be discharged while still under the residual effects of sedation (such as after a minor procedure like desexing) or a dental.

Set up a separate room for the cat to recover in before you re-introduce the two cats. The last thing a sick or groggy cat needs is to have to deal with aggression when he returns home. Place the cat’s food and water bowls, a litter tray and a comfortable bed in the room. Only re-introduce once the returning cat is feeling better and monitor closely for signs of aggression.

If a fight breaks out

Hopefully, if the introduction is closely followed this should not happen. But if it if does, take care. Never get between two cats who are fighting to avoid accidental injury to yourself. Throw a towel over the cats, clap your hands, or separate with a broom.

Safely move the returning cat to its own room, as outlined above and attempt to re-introduce again after 24 hours.

Signs of an aggressive cat:

  • Constricted pupils
  • Ears back
  • Whiskers
  • Hair on the back and tail standing on end
  • Hissing
  • Growling
  • Sideways stance
  • Tail hangs down
  • Makes direct eye contact with its target

Signs of a frightened cat:

  • Dilated pupils
  • Ears pressed to the side of the head
  • Tail tucked underneath the cat
  • Avoids eye contact

Do not attempt to calm down an aggressive cat, give them some space.

How to prevent non-recognition aggression

If you have experienced non-recognition aggression in the past, a slow re-introduction will be necessary.

  • If possible, schedule trips to the veterinarian or pet groomer so that the cats go together, so that all returning cats have the same scent, better still, look for a veterinarian who does house calls.
  • Look for a veterinarian who practices Fear Free or Cat-Friendly methods, which can make the overall experience for the cat less stressful.
  • If only one cat has been to the veterinarian or pet groomer, before their departure, grab a hand towel and rub the cat all over to transfer its scent onto the towel. Place in a plastic bag and tie it up. When the cat returns, move him into a room on his own and rub him down with the towel you used on him before his departure. Leave him alone in the room for 12-24 hours to allow time for him to groom himself and wash the foreign smell off him.
  • Once the cat has settled, open the door an inch or two to let resident cats sniff the returning cat. If all seems well, allow them to get closer and sniff each other under strict supervision.
  • Always make interactions positive when re-introducing cats.


  • Use cat treats or play to give both cats a positive experience while they become re-acquainted.
  • Remain calm; cats pick up on our emotions; if we are anxious, they will be anxious.

Does non-recognition aggression happen between cats and their human family?

Not that I have heard of, but many cat lovers can recall their cat’s nose being out of joint after a period of separation such as a holiday or a stay in hospital. This typically presents as the cat ignoring you, or going to the toilet on your bed or clothes instead of non-recognition aggression. We know that the cat does eventually come round; it just takes a few days of love and extra attention.

With patience, peace should return to the home, it sometimes takes a few days, but generally, once the returning cat has lost the foreign smell, and has recovered from the stress and effects of medication, the aggressor should calm down. If there has been no change after a week or so, speak to your veterinarian.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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