I’d better point this out before any of the woomongers notice it (assuming, perhaps wrongly, that any of them will know enough chemistry to notice) … mercury in tooth fillings is not in the form of a compound, it is an amalgam. That is, it consists of a solution of other metals (like silver) in liquid mercury which has the property of hardening to a solid metal.

A small amount of mercury (and the other metals) does leech out and get ingested, but the general medical view is that this quantity is too small to cause noticeable harm. You’d possibly ingest more mercury by eating a tuna fish sandwich.

That said, I always hated the taste of mercury fillings, and the pain they produce when you accidentally chew on a piece of foil wrapping, so when my fillings needed replacement I forked out some extra cash to have them replaced with non-metallic ones.

You are also mistaken in saying that ingesting a mercury compound is less harmful than metallic mercury, and your analogy with sodium chloride is flawed. Both sodium and chloride ions are essential in the body, but mercury ions are poisonous. In fact, a mercury compound is probably more harmful than the equivalent amount of metallic mercury, since it may be more easily absorbed, whereas metallic mercury will mostly pass through the digestive system unchanged. This is particularly true if the metallic mercury is immobilised in a solid material like a tooth filling, so a swallowed filling may well be found a couple of days later in the bottom of the toilet bowl.