Fluoride is the reduced form of fluorine that can cause a wide range of adverse, systemic effects.
The International Chiropractors Association opposes mass water fluoridation, considering it "possibly harmful and deprivation of the rights of citizens to be free from unwelcome mass medication."
Fluoride is a poison. If you choose to drink tap water, buy a reverse osmosis filter to remove fluoride and other contaminant and choose toothpaste made without flouride.