Some cat-to-cat introductions go smoothly, while others may take weeks or months before the cats co-exist with each other. The best way to do this is to go as slowly as necessary, and make every interaction a positive one. A strong long-term relationship is built on positive encounters. Once serious problems develop, it can be very challenging to remediate. Rushing an introduction can cause such serious problems. Most cats will learn to live in a multi-cat household, and some will enjoy it more than others.
Gradual introduction can help make each encounter a positive one. This can be achieved by having separate areas for each cat. Provide each cat with their own room, litter box, food, water, toys, and a safe place to hide (such as a cat carrier with a towel inside). If there is a resident cat, choose a room for your new cat that doesn't interrupt the resident cat's routine. Avoid direct interactions between the cats at first. Let the cats sniff each other under the closed door. Let them investigate and get used to each other's presence as long as there are no signs of fear or aggression. If that is going well, start to switch bedding and other items between the two cats. This way each cat can become used to the scent of the other without meeting face to face.
Watch closely for any signs of stress. If at any point you notice hissing, or either cat is growling, continue to keep them separate for as long as it takes for them to settle and be calm. Other signs of stress are: not eating, not using the litter box appropriately, over grooming, etc.
If the introduction is going well, a physical barrier should be placed to allow the cats to view each other without being able to make contact. If they start to sniff or even trying to play with each other across the barrier, you can begin supervised interactions. Open the barrier and let the cats come into the new room and explore at their own pace. Never force cats to come into a new territory, as this will make them increasingly tense and prolong the process. Let them explore each other's territory for short periods of time, then separate the cats and repeat this process a few times each day. The cats can also be distracted with toys or treats. This may not only distract them, but also serve as a reward for being together calmly. Never punish your cat for aggressive behavior toward another cat: it only ends up making your cats more stressed and will prolong the cat-to-cat introduction.
At the end of each positive encounter session, separate the cats. This allows them both to regain their sense of territory and confidence, which encourages another positive interaction at their next meeting. Continue this process daily, lengthening the amount of time they are together by a little bit each session. Supervising each interaction in the beginning ensures positive encounters and allows for immediate separation should the introduction not be positive. This should be done until they are tolerating each other's presence and you are certain that they will not harm each other.
This document created by the San Francisco SPCA with a grant from Maddie's Fund®.