As I understand it, you would show medical records about the medications you've used before and claim no treatment has been "satisfactory care". I'm just basing this on my own experience, similar enough to your own in that my diagnosis wasn't clear. I use it to manage pain and all I needed to say was that other medical options hadn't been sufficient.

There are articles in the Oregonian from time to time suggesting that employers don't have to accept your medicinal use and can fire you. Lots of people in Oregon, however, won't hassle you.