Mercury actually used to be a "cure" a long time ago. Of course, it really didn't do anything. Elemental mercury is so toxic that, when ingested, the body rejects it and it simply goes in one end and out the other.
The yogi didn't survive because of some magickal herb. He survived because the body is smart enough not to absorb elemental mercury. If you swallowed a bunch of mercury, you would probably survive too. But I don't recommend it.
The same phenomenon happens with just about all toxic materials. If you consume enough of it by mouth, the body will reject all of it (often by vomitting)