I have 2 barn cats (neutered males) that have always intermittently fought. These collars help when the fighting becomes intolerable. They may not get along perfectly but at least they do get along. I'm not thrilled with the initial fragrance of the collars and there is a powder that you can shake off but they do work. YMMV
"Works on stressed cat"
This collar works perfectly on an older stressed cat. After 5 years one of my male cats decided he no longer wanted to get along with one of my female cats. They still have to remain in separate rooms when I am not home but he no longer is fixated on her when I am at home. The collar took about 4 days to start working. I just purchased another 3 pack as this is the only thing that has worked for my situation.