Well, in pharmacology, if the effect is local, it's of course absolutely awkward to use it in any other way than as a local treatment.
Arvid Carlsson
In modern pharmacology it's so clear that even if you have a fixed dose of a drug, the individuals respond very differently to one and the same dose.
Fluorine has a protecting action against caries, but this is a local effect. If you drink it, you are running the risk of all kinds of toxic actions.