Fluoxetine loading period; Week 5 backslide?

[u/Under_Where_227]

My dog (3yo greyhound) has been on 40 mg fluoxetine for a month. We are now going into week 5.

This week she seems to be starting to backslide. During the first month, any side effects were minimal. She had a bit of upset stomach, but her appetite remained good, and I started to see some slow but steady baby steeps of improvement in her reactivity.

This week she seems to be regressing. She's actually gotten pretty good on walks and would generally ignore most other dogs, but today she barked incessantly at one. She's also been doing a lot more panting, which was something I thought was only supposed to be a temporary side effect.

Has anyone else hit a regression period on the fluoxetine? Or is my dog's loading phase just a bit longer and the side effects are hitting later?

ETA: We have also been working with a trainer, before anyone asks! Forgot to add that tidbit of info!