Two teenagers decided to lace mercury in cigarettes and smoke it! That was back in 80's and unfortunately they didn't live long afterwards.
As for lead, once gets in human body, it will NEVER come out.
As for Mercury, it is very highly restrictive materials, meaning it is almost impossible to get mercury legally, but for lead, it was close to being highly restrictive but EPA decided not to do so due to ammo requirements.
Seriously, Mercury is AWESOME metal! Very awesome! But, its vapor, unfortunately is very lethal.