No experience with phenobarb, but lots with prednisone. Sleet used to take pred for her seasonal allergies. In addition to usual side effects of extreme thirst and hunger, it turned her into a completely different dog. She was a snarling, miserable beast. Not full on aggressive, but most certainly not Sleet. For the last 3 years she's had depro-medral shots instead. Chemically they are almost the same drug, but Sleet reacts entirely differently to the depro. She stays her usual sweet self, just drinks more and stops scratching.
I've also had to give pred to a couple other dogs. They all got extra thirsty and hungry, but none of them got snarly like Sleet. Just like people dog's process drugs differently.
I would agree about the thyroid panel, if you haven't already done one. A full thyroid panel, not just a t-4 level. It'll cost more, but it actually tells you something. A t-4 test would not have diagnosed what is wrong with Earle. A full thyroid panel did. One of my clues that something was not right with Earle was that he was snarling at other dogs. Earle's never met a dog he didn't like so that was a clear sign something was wrong.
Also, have you eliminated the possibility that Josie is in pain? I ask only because my Goldie used to be randomly dog aggressive. No pattern to it, she would be playing, perfectly happy and then suddenly attack. After an extra complete exam, including every blood test known to man, we found that Goldie was actually in pain. She had been hiding it very well. When she moved a certain way, her shoulders hurt and she would lash out at the closest dog thinking that they had hurt her. She's on an anti-inflammatory pain killer every day for the rest of her life and she's a different dog. She's still fussy about her friends. She will never like Preacher or other females, but she lives with another dog; she gets regular time in the big yard with the main pack, and I don't have to worry about her if we run into a dog on the trails. She's a much happier girl.
Good luck with all of this.
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