Botulinum, I'd guess (among natural toxins). A casual Google seems to indicate I'm right.

Yup. 10x more toxic than palytoxin.

And we inject it in ourselves voluntarily.

Humans are such funny creatures.

There is a poisonous dosage of ALL chemicals. H2O among them. The question is, is there a dosage of botulism that has a therapeutic effect?

Botulinum toxin type A is known as "Botox." It has some therapeutic uses, but mostly cosmetic ones.

http://en.wikipedia.org/wiki/Botulinum_toxin

Palytoxin.

EDIT: Oh, the deadliest, not the second deadliest. Botulinum toxin, of course.

That is the second deadliest poison, not the deadliest.