Short answer: probably nothing.
Longer answer: Two guys I work with asked me yesterday what would happen if they each took a shot of mercury. Unfortunately I was in the middle of addressing a production issue, and couldn’t really answer, but it was a fun question, so here’s the answer…
Elemental mercury (quicksilver) isn’t absorbed very well by your GI tract. In fact, it appears that less than 0.01% of whatever amount you consume will be absorbed, assuming you have a healthy GI tract. Conversely, this means 99.99% will be excreted without it reaching your bloodstream. So a shot of liquid mercury is unlikely to do you any lasting harm, though I wouldn’t recommend it.
Much more dangerous is aerosolized mercury which is readily absorbed in the lungs, where absorption rates reach 80%. Also dangerous is methylmercury, an organic compound. (This is the mercury that you’ll find in fish.)
(One of these days, I will import all of my old pharmacy blog posts into the archives…)