All I can think of is Clomicalm, or a similar tranquilizer. Clomicalm is given to dogs, but is very well tolerated in cats. What was she on before?

Perhaps a second opinion from another vet would help.

And maybe another diet change.

I know you will get a lot of suggestions here!

Check Google on this too. I read one story where it turned out the kitty was allergic to a perfumed kitty litter!