For over 70 years, communities in the United States have benefited from community water fluoridation. Water fluoridation is the process of adding a small amount of fluoride to public water supplies to a level known to make teeth stronger and prevent cavities.
Almost all water contains some naturally-occurring fluoride, but usually at levels too low to prevent tooth decay.
Fluoridated water is effective, because it keeps a low level of fluoride in the mouth, specifically in the dental plaque and saliva, all day. Even with the use of other fluoride products, such as toothpaste and mouth rinses, fluoridated water reduces tooth decay by 25% among children and adults.
Tooth decay is caused by certain bacteria in the mouth. When a person eats sugar and other refined carbohydrates, these bacteria produce acid that removes minerals from the surface of the tooth. Fluoride helps to remineralize tooth surfaces and prevents cavities from forming.
So, go ahead, fill up your glass or refillable bottle from your tap.
Source: Centers for Disease Control and Prevention https://www.cdc.gov/fluoridation/basics/index.htm