|Many owners of multiple cats have noticed that when one cat goes to the veterinarian and then returns home, she is often greeted with aggression by the other cat(s). The sense of smell is extremely important to cats. So although the cat that came back from the veterinarian looks the same and sounds the same as before she went to the veterinarian, she does not smell the same to the cat(s) who stayed home. Thus, the stay-at-home cat hisses and may become aggressive, sometimes batting at the other cat, since she seems like a stranger.
To try to correct this cat aggression, get a clean towel and rub it on the cat who stayed home. Now rub it on the cat who went to the veterinarian, and then back on the cat who stayed home. This gets the scent of each cat onto the other cat. If the cats smell alike, the stay-at-home cat is less likely to get upset. As an alternative, some people take both cats in the same carrier to the veterinarian, even if only one cat is being checked. The cat who remains in the carrier will then smell like the veterinarian's office too, and if the carrier is situated right, may never even lose sight of the other cat.