Hang on, so I could totally sell off-patent insulin in America if I wanted to with a “not for therapeutic use” label? How do all those people keeping dying “because they couldn’t afford insulin” then? Feels like someone would capture the cheap end.
The issue is getting that off label insulin distributed. Insulin must be refrigerated which really makes distribution difficult. A pharmacy isn't going to store it in their refrigerators and stores aren't going to plop it down next to the milk.
Then, of course, you run into the risk that you get sued anyways by current insulin manufacturers for patent violations (good luck with that!).
Oh, and you also run the risk that someone takes your insulin and sues you for a bad reaction. With FDA approval, there's a much lower legal risk.
And finally, after you setup your insulin product, you simply run the risk that nobody uses it because it's off label and their doctors didn't prescribe it.
Oh, and for certain, your insurance will not cover it at all. So, bunch of risks with minimal rewards. Still completely legal. Can be done doesn't mean will be done.
Insulin is a FDA approved substance with therapeutic properties. How you label it is irrelevant.
You can't sell heroin with a "nutritional supplement"label either.
If a substance has no classification(neither controlled nor therapeutic) however, like water for example; you're free to sell it, to treat dehydration, or put a homeopathy label on it and sell it to cure cancer if you prefer.