Like in some tribal society. Someone in position, like a folk healer, but is an expert at poisons and poisoning.
From a historical stand point,
Apothecaries dispensed viles or poisons as well as medicines, and as is still the case, medicines could be either beneficial or harmful if inappropriately used.
If you've ever read Romeo and Juliet, you may recall that Romeo purchased his vial of poison from an apothecary illegally.
If you are looking for the modern definition:
a druggist; a pharmacist.