Under the Federal Food, Drug, and Cosmetic Act, whether a substance is a drug depends largely on its intended use. FDA's regulation at 21 CFR 201.128 defines intended use as the objective intent of the persons responsible for the product's labeling.
That intent may be shown by their expressions, the design or composition of the product, and the circumstances surrounding its distribution, including labeling claims, advertising, oral or written statements by them or their representatives, and uses they know of but do not label for. A disclaimer is one piece of that evidence, weighed against the rest.
How FDA applies this test is a matter of agency interpretation, seen mainly in its warning letters. That interpretation guides current practice, but it is not a court ruling, and it, like the regulation itself, can change.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.