Fluoride is one of the more controversial ingredients in toothpaste. According to the ADA , fluoride is the most important ingredient for healthy teeth: It strengthens enamel, prevents cavities and fights plaque. The debate surrounding the safety of fluoride stretches back to the 1950s when the move to add fluoride to drinking water began. Fluoride ingested in unsafe levels is toxic to humans, and the FDA mandates warning labels on all products containing the chemical that reads: "Keep out of reach of children aged 6 and under. If more than is used for normal brushing is swallowed, contact your physician or local poison control center."