Rick Collins "The Steroid Lawyer" discussed this in a video. Intent is the critical element.
Label it as the branded drug, it's a "Counterfeit pharmaceutical". Sell it as intended for human use, it becomes "Introducing a misbranded (or unapproved) drug into interstate commerce.".
The FDA recently shut down two online sellers who used the "for research use only" technique for Sema and Tirz, yet provided instructions on reconstituting and dosage. That was enough evidence.
I can buy pure minoxidil, legally, all day long. Formulate it into a shampoo for personal use, there's nothing to prosecute. Press it into tabs and start selling it, I'm going to have a problem with the FDA.
Urea is perfectly legal for a variety of uses, until you make it into a prescription strength foot cream and sell it, or perhaps, in theory get caught possessing it in "Rx only drug" form, but possession of a non-controlled prescription drug for personal use gets prosecuted about as often as possessing a controlled drug prescribed to you beyond the "discard by" expiration date on the bottle. Yes, possessing the unused codeine from your dental appointment 3 years ago is illegal, technically, as the prescription is no longer "valid" at that point.