Originally Posted by
sandmote
I do assume mercury gets labeled by Detect Poison, or whatever other equivalent you'd have available.
Second, humans are omnivorous and most of the meat we eat is from primary consumers. There's a reason mercury is considered a bigger problem in tuna than in venison. When you eat something containing mercury, it builds up in your system. Particularly if a creature is eating a lot of large fish, its mercury levels are going to build up a lot faster than they do in a typical human. And of course dragons also live a lot longer, so there's more time for it to build up.
Of course, all of this can be hand waved for a game setting. I just figure a lot of parties are going to start wondering about what a dragon (particularly a metallic) is doing with a bunch of mercury.