You're right, the same dealer selling opiates probably wouldn't not sell this new drug, at least not to a regular customer (gotta keep those customers hooked). But other dealers might emerge that would (helping people get around the embarrassing trip to the doctor or detox center). Certainly overseas pharmacies will find a way to get their hands on it. You can get all sorts of proscribed meds from them without a prescription.
I don't know that this is a big deal, or even that it could be prevented even if it is.