I don't understand what Mattes was getting at, but to the best of my knowledge, much of what whoever sent that in wrote was simply wrong.
over-the-counter sind Arzneimittel, die nicht in der Freiwahl erhältlich (keine Selbstbedienung) sind.
That's simply not true, as far as I know. OTC has nothing to do with a literal counter or with who serves you. Yes, the literal, etymological origin of the phrase 'over the counter' refers to a counter in a store. But I assume the reason for that was simply that that's how all stores used to function long ago -- you asked for help, and every item you requested was handed to you over the counter. The whole idea of self-service stores, where you didn't ask a clerk but simply picked up the item yourself, was a later historical development.
As for the German words, I don't want to read the original law, but is the point just that in German there are actually three levels, not two? If so, then you might just have to add a description for the middle one, as a cultural difference for which there's not necessarily a direct English equivalent.
rezeptfrei, freiverkäuflich = OTC, nonprescription
apothekenpflichtig = (sold only in pharmacies)
rekeptpflichtig, verschreibungspflichtig = prescription
At least not in AE. There may be a middle level in BE, some official term that corresponds to 'sold only in pharmacies'; wasn't there an abbreviation or something that came up just the other day?
In fact, OTC might be basically an AE term anyway, I don't know. And I haven't taken time to look all this up to verify it, so this is only based on my experience. But I can't imagine that there would be a law like that restricting where you can sell something that wouldn't be fairly common knowledge. If there were, a lot of other people would also be unaware of it, because I'm sure that most people understand OTC to mean simply nonprescription.