feliway
It seems to work, I would have been sceptical about it before, but we were desperate and prepared to try anything! We have an adult cat who is not keen on the new kitten, to put it mildly, her reaction is very odd, and this definitely seems to calm her. We got the spray too, though it's not stocked anymore. I would say give this a go, it does seem to help, and it's better value here than in our local vets.