Fluoride is a naturally occurring element.
All water contains some naturally occurring fluoride.
Fluoride can help prevent tooth decay by being used systemically and topically. Topical uses include brushing with fluoridated toothpaste or using fluoride mouth rinses. Systemic use refers to the ingestion of fluoride through fluoride supplements or by drinking fluoridated water. There is no chemical difference in the fluoride that is provided by either natural or adjusted means. And there is no difference in the dental benefits of water that contains the optimal level of fluoride whether the fluoride is naturally occurring or added.