There is one possibility: your lethal substance is an ingested poison. With a low dose, it goes into the bloodstream and kills you. In a large dose, it is so harmful your estomach immediately throws it up, saving your life.

It isn't pretty, but it is reason some suicides with pills failed. And as another example, Napoleon tried to kill himself during his exile and he took so much poison he threw it up and survived (to be murdered by another poison later).

There is one possibility: your lethal substance is an ingested poison. With a low dose, it goes into the bloodstream and kills you. In a large dose, it is so harmful your stomach immediately throws it up, saving your life.

It isn't pretty, but it is reason some suicides with pills failed. And as another example, Napoleon tried to kill himself during his exile and he took so much poison he threw it up and survived (to be murdered by another poison later).