"safely" - the metal would be strictly beneficial; there would be no health problems caused by it (perhaps at least for most, there may be "birth defects" which cause some of the population reject it. I'm not set on that idea though).
"functional" - I mean to use as strength enhancement/armor. Preferably, it would be enough to drastically slow a bullet, reducing the damage caused.
"metal" - I was thinking about iron, such as a beavers teeth. However, if you can justify something else, then by all means.
This question is merely asking how much more metal (pure or compound) a creature would need to be able to resist a bullet, or stop sword. would that concentration be unhealthy to life as we know it?