That's essentially untrue in developed nations. Everyone has gotten a prescription at some point.
Unless you mean to say that drug companies can advertise to prescription drug users for the specific window when they happen to be actively taking a prescription drug. Which seems rather difficult to manage and would require a massive privacy violation to achieve. Plus I'm not clear what the delivery mechanism would be unless you're just talking about inserts delivered with the drugs themselves.
What that means is that the particular Rx drug being advertised can only be advertised to consumers of that Rx drug in question. So insulin can only be advertised to people with diabetes.
The interesting thing about this is that it leaves most of the power with the HCP.
As for the delivery mechanism, there weren't too many options in the recent past, however, these days there are POC marketing companies like Outcome Health (and others) and a growing number of software applications are aware of their user's health condition / needs (with permission of course either explicitly or by virtue of the intended use, like apps for diabetes patients, etc.).
However, many countries allow for Rx drug advertising / marketing targeted towards consumers.