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.

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.