This is a very interesting question with a not so simple answer. To answer the last part of your question: Yes, fluoride is good for you teeth. Fluoride helps prevent the breakdown of minerals in your teeth. When fluoride is absorbed into your teeth (through drinking water or toothpaste) it actually attracts other minerals, like calcium, to your teeth where they form mineral deposits. These mineral deposits strengthen your teeth and prevent tooth decay. Fluoride also helps kill bacteria that live on your gums and teeth so that they don’t eat away at your teeth and cause damage.

As far as chlorine goes, it is a larger element than fluorine and therefore has different chemical properties. Chloride has not been shown to draw calcium into your teeth like fluoride, due to the difference in atom size and electronegativity. So for all these reasons, we add fluoride to toothpaste and drinking water, but not chloride.

Now back to the first part of your question, fluoride can be bad for you if ingested in high enough doses. The most common negative side effect of ingesting too much fluoride is something called “fluorosis” This is when too many minerals are attracted to your teeth or bones. When this happens, your teeth may begin to get dark stains from the mineral deposits. It may also cause your bones to break slightly more easily. Typically these are not major concerns, and for most people a high enough level of fluoride will never be ingested from drinking water or toothpaste to lead to these problems. As is the case with anything, ingesting enough fluoride is toxic and you could die. However, you would have to be ingesting a TON of fluoride, at amounts hundreds of times greater than the maximum dose found in drinking water and toothpaste. I’m sure you don’t worry about things like perfume, nail polish, and lipstick, but inhaling/ingesting any of those things at a high enough dose is also incredibly toxic (even deadly).

So, while it is true that extremely high doses of fluoride are toxic and could potentially kill a person, this is true for just about any substance on earth. Remember, moderation is key, and the amount of fluoride in drinking water and toothpaste is extremely moderate. The addition of fluoride to drinking water has even been described by the Center for Disease Control as “one of the top medical achievements of the 20th century”. So go ahead and enjoy your Sensodine and those cavity free teeth you’ll have as a result.