So I'm kicking around an idea for a work of fiction.
Does anyone know chemicals/poisons/etc. that would work like this:
Scenario 1
If the body ingests A, it is harmful.
If the body ingests B, it is harmful.
If the body ingests A & B, the substances are neutralized.
Scenario 2
If the body ingests A, it is harmful.
If the body already contains B, then A is ineffective.
If there are many examples, what is a good example related to something a murderer would try to use?