In case anyone else has this problem, I want my experience to be available to them. There was very little to read on the DURATION of initial phenobarbital side effects on the web, and I was very frightened by what I saw. So, here's what happened with my dog.
The side effects (which were severe) lasted approximately three weeks. It is possible that they can last this long. Most people say they go away in a day or two, but in very sensitive dogs, it takes much longer.