Fluoride in toothpaste comes as either sodium fluoride or tin fluoride (marketed as 'stannous' fluoride). Both are, as a dentist might say aniti-carrious; the tin does a better job of hardening enamel, but can cause joint issues if you swallow it. Simple solution: do not swallow toothpaste.

There are some better than tin fluoride enamel hardeners available outside the US, but not here (I've gone on about this elsewhere).