I feel like I am reading my own post. I recently got an externship in the Emergency Department as well. I was diagnosed with ADHD when I was in high school, but have just started taking medication for it a little over a year ago. Before, the fact that I couldn't pay attention never affected my grades but once I began nursing school it was a different story.
I also had to get a pre-employment drug screen and physical. If you can prove that you are prescribed these medications it should not be a problem. I mean having ADD is not something they can fire you over. That would be discrimination which is illegal. So I would bring your bottles so they can make copies of them, and then that should be it.
Best of luck on your externship! I am sure you are excited. I know I am!!