Larry,

I've been in your shoes. Not because of an aggressive issue, but because one of my cats was terrorizing the other.

My vet suggested some Amitryptiline. Casey was just not the same on that stuff. He became withdrawn and cranky and was hiding in the closet. I didn't like the way it changed Casey's personality. So I rehomed Tucker.

There are all natural homeopathic remedies (flour essences) that my friends in rescue use. They work without the horrible side effects.

Good luck.