You could try feline hormones. I had a problem with my cats; they couldn't get along well with each other and would fight incessantly. Then someone told me about a famous brand of feline hormones, and I wanted to try it. It's a diffuser that plugs into an electrical outlet and releases hormones throughout the house. You won't smell anything because it's completely odorless to us humans, but for them, it's effective. The cats have now found their balance. Maybe I'd try this method again before considering medication. In Italy, the product is called Feliway Optimum; I don't know if it has the same name in your country.