The medical industry wants to control it.
So they try to call it a drug.
It is not.
In this case it is a vitamin being used as a drug, specifically for the prevention or cure of a disease, which fits the definition of a drug.
Vitamin supplements for poor diets are supposed to reflect an amount meeting physiologic requirements.
When something is given in pharmacologic doses to treat a precancerous lesion, in this case leukoplakia, this vitamin is now a drug because of the dose.
I'm not a pharmacist, but I had to study pharmacology.