Places looked: Wiki, Google with search words 'slow-acting poison', metallic poisoning, research on Thallium and arsenic
I found one post here that relates to slow-acting poisons, but some details that we planned for our story don't fit, so I hope to get further answers here.
As planned, anyway: The poison should be something which shows effects slowly, the person knows she's being poisoned turns to a doctor for help finding an antidote. (That's why most substances found didn't work as it was mostly clear what the treatment would be, and it had to be immediately.
The antidote has to be found yet, or is very hard to get. There should be no long-lasting life-threatening effects once it's given.
Is such a thing possible?