I think the only in-game "chem" (not drugs... Drugs are bad, Mkay) that has the right effect would be the Healing powder, although it takes a large dose. If you want to keep with in Canon items, maybe it could be a custom mix using either (or both) Xander roots or Broc Flowers (the key components of Stimpacks and Healing powder).
But, I think this is an area where sticking to in-game chems isn't required. I see no reason why chloroform or other real world tranqs couldnt reasoinably exist in the fallout world (those auto-docs have got to use something as a general anaesthetic).
I don't think I could use healing powder... That seemed to me to be something used almost exclusively by tribals and the few travelers they trade with. I will probably use chloroform at one point or another, but something that can be injected with a needle is what I'd like to use for the pilot chapter.
Using a real workd substance hadn't even crossed my mind, dunno why... I suppose I'm gonna have to look into some of this stuff now. Probably gonna write chapter one tonight and see what the response is.
EDIT: I had to change around some things but chloroform got the job done. The fanfics pilot chapter is up. A mod can lock this thread if they like.