Do you eat your soap? Do you drink your shampoo?
No? Oh, so you can tell the difference between topical application and ingestion.
To apply fluoride topically to the teeth and gums, and then spit it out - as in brushing with fluoride toothpaste - has been shown to reduce the incidence of cavities.
However, the ingestion of fluoride has not been proven to reduce cavities, has no benefit for human health, and has been known for over 50 years to be toxic. See The Fluoride Deception for extreme documentation.