Greetings: New here, have pressing issue... Once friends, now cats hate each other...

I have two cats, The Zoops a 1.5 year old female, part siamese I think. She has been fixed for about a year. Mr. Squeaks is an approx 6 month old male. When we brought Mr. Squeaks into the home there was a short acclimation period for both cats. After that they were very good together, washing each other, playing, eating together.

This past Monday Mr. Squeaks was fixed. When he came home The Zoops was not happy. I chaulked it up to Mr. Squeaks having the odor of the vet/other animals on him. Things were strained between them for Monday afternoon and night. Yesterday it seemed to get better, they lay near each other on the floor and even played a little bit.

Today was vastly different. The Zoops is very vocal in her displeasure with Mr Squeaks. She growls and hisses at him. He tries to get close and The Zoops runs, and he gives chase. He will not give up and I've had to segregate them. I'm bloodied and frustrated. If I leave them segregated in different rooms they cry and claw at the door and wall.

I don't know what to do...

You are going to need to do a reintroduction. Review this link for ideas. Keep them separated for awhile. Try feeding them treats on both sides of the door. Let them get familiar with each others scents and try the vanilla trick where you put some real vanilla extract on their chins and near the base of the tail.

You can also look into a Feliway diffuser and some people have had success with Bach Rescue Remedy (a homeopathic treatment) in the water.

This is quite a problem. I am facing the same with my 2 1/2 years old cat Milli not accepting a stray boy cat [younger than her]. And getting familiar with each others scents doesn't work for us because Millie doesn't even need to feel his scent, it is enough to see him outside, in the distance, and she goes mad. I don't know what you could do, but definetly you can't keep them locked in separate rooms for the rest of their lives! Don't give up yet.

I have noticed this too after cats came back from the vets. They smell different and as a result, their friends don't recognize them

Try putting a little bit of vanilla extract (the real thing) under each one's chin and on the base of their tail on their back. That will make both of them smell the same and may soothe the issue for a while until the resident cat realizes that this is his/her friend again.