Arvid Carlsson — Swedish Scientist born on January 25, 1923,

Arvid Carlsson is a Swedish scientist who is best known for his work with the neurotransmitter dopamine and its effects in Parkinson's disease. For his work on dopamine, Carlsson was awarded the Nobel Prize in Physiology or Medicine in 2000, along with co-recipients Eric Kandel and Paul Greengard... (wikipedia)

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.
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.
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.