Labeled as a "non-toxic etc." injectable in a vial? If I were customs I would be suspicious about anything in a vial, regardless of label. Besides, most every injectable that can be bought without a prescription is water-based and it typically a vitamin or mineral. But I don't know all the details and I'll leave that to you and your source.
I do know that there is a new law in the US last year that made buying prescription drugs from overseas companies illegal.
It was because the US companies were losing too many profits, though I think they threw something else in there like safety or whatever so they wouldn't look so greedy. Even if you have a prescription you either can't order prescription drugs at all or for more than a 30 day period, I don't remember exactly.