Can you put a baby gate up at the top of the stairway for awhile?
We hung a bell on a cord over the handle of our slider door and taught our dog to ring it with her nose when she wants outside. If you want to know how to do this, pm me and I will let you know. Our vet told us that our dog was most likely trying to let us know that she needed out to do her business, but that we weren't there to see it. The bell worked like a charm. I can hear her ring it almost anywhere in the house.
Good luck!
I am glad to hear that your dog is doing well after the whole medication scare.