While fluoride is a naturally occurring compound, it can still cause side effects when consumed in large doses.  In 2015, the U.S. Department of Health and Human Services (HHS) released its recommendation lowering the optimal concentration of fluoride to 0.7 mg per liter of water from the previous 1962 of 1.2 mg per liter.

Dental fluorosis happens when you consume too much fluoride while your teeth are still forming in the gingiva (gums). This results in white spots on the surface of the teeth. Other than the appearance of white spots, dental fluorosis doesn’t cause any symptoms or harm. It affects only children under the age of 8 who have unerupted permanent teeth. Children are also more likely to swallow toothpaste, which contains significantly more fluoride than fluoridated water. Children’s risk of developing dental fluorosis can be reduced by supervising them when they brush their teeth to make sure they aren’t swallowing large amounts of toothpaste.