What happens if an adult uses too much flouride for their teeth?

Flouride : Normal applications of flouride will enhance the hardness of the enamel. I am assuming you mean traditional flouride applications like in varnish form or tooth pastes. Flouride is toxic and is generally a bad idea to be ingested.

No harm: Using too much Fluoride on your teeth will not harm them. It is more of a concern that you are not swallowing the fluoride. It can be toxic or at least nauseating. There is a limit to what prevents decay and lessens sensitivity. Use as prescribed.

Good for teeth: Bad for the body. Do not swallow fluoride.

Nothing: I do not see any side effect, except you my digest some which is that is bad.

Absolutely nothing! : Assuming the toothpaste is not swallowed, Fluoride will not harm him/her nor will it make teeth any stronger. Fluoride, provides a very commercially "hyped up" slight remineralization (hardening) of slightly decayed teeth. More Fluoride won't make your teeth stronger! try something that actually stops tooth decay and gum disease ...Floss daily!