I am not sure of the exact mechanism of how it works as an antidote to cyanide-but it does have other medical uses to, also it is contained in food products, but in all these cases-the quantity is much much smaller-so it will not have the same effect-the idea with the dosage is that it works as a poison would by flooding the body with so much of a toxin it can not fight fast enough to expel it. There are probably other medical antidotes that if usage exceeded the recommened dosage would become harmful rather than beneficial- just not all of them will be lethal. I do know what you mean though- it does seem kind of a strange fact that one 'suicide salt' could counteract another one.
