If your cats suddenly start fighting after living together without altercations, the first step is to take them both to your veterinarian for medical evaluation. Medical problems can cause your cat to be painful, grouchy, or confused, increasing the likelihood of aggressive behavior. If a medical evaluation reveals no abnormalities, the following recommendations may help them resolve their differences:

• Separation: The first step is to separate the cats from each other completely. Place a barrier at the bottom of the door or confine them in separate parts of the house where there is no opportunity for contact. Provide each cat with food, water, a litter box, a comfortable place to sleep/rest, and regular interaction (playing and/or petting). While the cats are separated, create a list of ‘triggers’ that cause fights. For example, your cats may fight because both want attention from you, they may also fight over preferred resting places or when they see another cat outdoors. If the cats are primarily fighting over resources, such as access to food or resting places, provide them with abundant resources so that it is impossible to guard all of them at once. If the cats attack each other after seeing another animal outdoors, block visual access to the outdoors. Opaque ‘wallpaper for windows’ works well in this situation.
• Spot the Signs: If the cats are fighting over access to and attention from you, teach them that this behavior is unacceptable. You can do this by paying close attention to the warning signs of anxiety and aggression. These signs can include dilated pupils, direct staring at the other cat, tense body posture, and a swishing tail. When you see these signs, get up from what you were doing and leave the room. The cats will now have nothing left to fight over. It is crucial that you do this technique EARLY and at the first signs of anxiety. If you wait too long, the cats will not remember why they started behaving aggressively. They will only remember what they are doing and likely behave aggressively toward the other cat.
• Distraction: Another technique you can utilize to prevent a fight is interruption and redirection. When you see the first signs of anxiety, distract the cats with a loud noise, such as clapping your hands or dropping a book on the floor. Keep a box with ‘special’ toys or treats that your cat really likes. After distracting the cats, call them to you and reward them (for coming when you called) and play with them individually or feed them a food reward.
• Reintroduction: After two weeks, or once you have created your list of triggers and the techniques that you will use to prevent aggressive events, it is time to reintroduce the cats to each other. Start by allowing the cats some access to each other, such as smelling each other through the crack at the bottom of a door. Next, start exercises where you give each cat a small bowl of special food five feet away from a door adjoining two rooms. Canned food or a small amount of tuna works well. The more frequently you do these exercises, the more quickly your cats will progress, but aim for a minimum of twice daily. After the cats eat for a few seconds, open the door between the rooms.
• If the cats show warning signs of aggression, close the door immediately and start your next exercise further away from the door. Over time, move the cats closer to the door and to each other until the cats are able to eat side by side without behaving aggressively. You are teaching them that good things happen when they behave well in each other’s presence. If the cats show warning signs of aggression during any of these exercises, end the session immediately, remove the food and reformulate your plan to prevent a reoccurrence of aggression.
• Feliway®: Cats have glands in their cheeks that produce pheromones. When your cat rubs their cheeks against a wall, chair, or your leg, they produce pheromones, which are chemical substances that can help to relieve anxiety and provide information about the cat who is producing those pheromones. Some cats respond very well to Feliway®, a synthetic pheromone spray/diffuser that can be bought online or in pet stores. Feliway® is sometimes effective in reducing the occurrence of aggressive events due to its calming effects.
• Supervised Interactions: After two to three weeks of feeding exercises with no aggression, you can start allowing the cats direct access to each other with your supervision. Keep each room of the house equipped with special toys and treats. If the cats show warning signs of aggression, distract them, redirect their attention toward you and take note of when and why the behavior occurred. Remember that mild aggressive displays, such as hissing and walking away, are normal when two cats have not seen each other for several weeks.
• Teach Recall: If one cat seems to be bullying another cat, first make sure that the ‘bully cat’ is getting enough attention/interaction. Secondly, teach the cat to come to you when you call its name. Most cats already ‘know’ this behavior. They ‘come’ when they hear the treat container making noise. Start saying ‘come’ immediately after shaking the treat container. Your cat will soon learn to associate ‘come’ with treats. You can now use the cue ‘come’ to redirect your ‘bully’ cat’s behavior. If you see them starting to stalk your other cat, tell the bully to ‘come.’ When they come to you, play with a special toy or give them a special treat to reward them. This teaches the bully cat that they get rewarded when they come to you instead of behaving aggressively.

If these techniques do not solve the problems between your cats, please seek the help of your veterinarian or a behaviorist. Remember that cats take time to learn how to behave aggressively. It will also take time to teach them NOT to behave aggressively. Be patient and creative. In most cases, providing them with adequate playtime/social stimulation, preventing aggressive incidents from occurring, and employing desensitization and counterconditioning will resolve the problem.

We are always here to help! You can find additional behavior and training resources at pasadenahumane.org/behavior.