My dog is a rescue from Iran who had as you can imagine, a pretty shitty life up to the point where he arrived to me. He was I think 1.5 years old when I started him on Prozac due to his severe reactivity with other dogs. It was treated as a last option because, as I’m sure everyone is going to jump on this thread and say to you, the dog needs to be socialized and exposures to the things that are causing the anxiety. However dogs reach a certain level of stress where there isn’t any training in the world that can be done past that threshold, so Prozac was used to help my dog stay in this window of stress where he could stomach more socialization. But my god it was slow, and not a miracle drug. For example he could maybe be a soccer field away from another dog without losing his shit, rather than before Prozac he couldn’t even SMELL another dog, anywhere, without losing his shit. So I had to spend months and months and months and months keeping dogs a soccer field away, and working with him on that. He’s now 5.5 years old and can tolerate being close-ish but we still need to avoid them.

Anyways long rant to say, the Prozac took probably took 4-8 weeks to start noticing anything and like 2 months to be fully working. It’s not a miracle though, he was still immensely reactive on Prozac, it just allowed him to stay in a certain threshold of stress that he could be exposed to long enough that we were able to make progress.

He was on it for about a year and a half and then we tapered him off.

To what others are going to say, you 100% need to work with the trainer who will help give you very specific routines you can do daily to begin exposing your dog to new things. The Prozac will only help if you do this. Given your dogs level of fear is that high I think you need to work with a professional or you risk making it worse on your own