. . . what is the Death Serum?

I just noticed on my blog statistics that there's usually at least one person somewhere in the world that is on this blog. So, you, yes, YOU--I have a question for you. But only if you've read Allegiant.
Is the Death Serum just poison? Because I know that it sounds so cool in the book. DEATH SERUM. But isn't is just poison? Wouldn't they at least have the same effects? The Death Serum kills you and poison kills you . . .
Just a thought.