Fluoride helps prevent tooth decay, but too much of it can be harmful. Too much fluoride in water for children less than eight to nine years old can cause dental fluorosis, which is a discoloration of the permanent teeth. This condition may be unsightly, but can be treated by cosmetic procedures. Significantly higher levels of fluoride use over time can cause bone disease.
Fluoride toxicity can occur in rare instances if fluoride is ingested in large amounts over a short period of time, most commonly due to accidental overdose of fluoride supplements. Fluoride toxicity can cause diarrhea, nausea, abdominal pain, vomiting, increased salivation, or increased thirst. These symptoms start half an hour after ingestion and may last up to 24 hours.