I had a fear aggressive large breed in the 90’s who was put on fluoxetine by a veterinary behaviourist, as part of a comprehensive behavioural programme. Back then we used to have fetch it from the local pharmacy ourselves, via veterinary prescription, as vets weren’t allowed to keep it in stock, but having a dog on another ‘human’ prescription medicine at the moment, that is dispensed by our vets, so things may have changed. Back then it was about £150 for 28 days, iirc, but I have no idea what a similar med would cost now.
He was completely unable to engage when confronted with his triggers before he went on it, which is why we had to go down that route, essentially it was trying the fluoxetine + behavioural programme or euthanasia. (We had already tried working with a qualified behaviourist without meds, unsuccessfully.)
We had weekly behavioural sessions with the supervising vet, then daily exercises to carry out at home. It definitely helped. There was no way we would have made the progress we did without it, but it wasn’t a ‘cure’ and it involved a huge (and expensive) commitment to a massive behavioural modification programme.
Our boy was both dog and people reactive and very big and strong and he never reached a point where he liked unknown people or dogs (although he did love other dogs brought into our family) but he went from uncontrollable to being able to walk calmly past and ignore the things that used to trigger him and we were able to gradually introduce him to people and dogs that we needed him to be comfortable around and he would then accept and be friends with them.
He didn’t remain on the fluoxetine long-term, it was specifically part of the behavioural programme and he was carefully weaned off it when he had made sufficient progress. We then used the behavioural management techniques we had learned, plus environmental management to manage him from there (ie he was always muzzled in public and we had a kennel and run for him at home for when people he didn’t know were coming to the house).
It’s definitely not something to be taken lightly. It’s a huge commitment and imo should only be used if you are working with a highly qualified behaviourist, preferably a veterinary behaviourist or both in collaboration.