Anyone With Experience Giving Mirtazapine to Dog?

Our vet put our 14.5-year-old dog on Mirtazapine yesterday to improve her appetite. (She has some liver issues and had Hemangiosarcoma in her spleen, which was removed five weeks ago.)

It definitely improved her appetite. It also turned her from a 14.5-year-old into a 14.5-month-old. In the few hours after medicating, she becomes very active, wants to go on five walks an hour, tried to go upstairs, ran up a hill, etc. In short, it's like the dog version of the movie Cocoon. These things are definitely Puppy Uppers.

I've read up on Serotonin Syndrome and she doesn't appear to be having that -- no shivering, no quaking, no trouble breathing. She's also not on any other meds that would affect her Serotonin; from what I've read about Serotonin Syndrome, that only occurs with Mirtazapine if the dog is being given another drug that affects Serotonin levels. (I also contacted the vet's office so that's in motion.)

I'd be interested in hearing if anyone else has had experience with a dog becoming somewhat hyper on Mirtazapine -- specifically, does the dog become more and more hyper the longer you medicate, or will this taper off after a while?