The concept I'm trying to capture is a cure or medicine which can also be poisonous. An example which comes to mind would be treating syphilis with mercury, or perhaps chemotherapy for cancer treatment.
The thing that is both poison and cure is called a pharmakon. (a term from ancient Greek)
The term "pharmakos" later became the term "pharmakeus" which refers to "a drug, spell-giving potion, druggist, poisoner, by extension a magician or a sorcerer."
A variation of this term is "pharmakon" (φάρμακον) a complex term meaning sacrament, remedy, poison, talisman, cosmetic, perfume or intoxicant. From this, the modern term "pharmacology" emerged.
Chemotherapy (or chemotherapic/cytotoxic subtances) is a pharmakon also. Because "chemo" indicates the toxic chemicals and "therapy" indicates the cure.
Here is a passage from "The Routledge Handbook of Language and Health Communication" edited by Heidi Hamilton, Wen-ying Sylvia Chou:
There is an adage from Paracelsus (sometimes called the father of toxicology):
Dosis facit venenum.
(The dose makes the poison.)
A drug. (Seriously; I'm not messing about with different senses.)
A medical spokesman, responding to criticism about certain drugs, pinpointed the dilemma:
'Show me a drug that's effective, and I'll show you a drug that has side effects.'
A more recent paraphrase can be found here.
Hormesis: Providing stimulus by nontoxic amounts of a toxic agent. If it is self-administered -think of Princess Bride- it is a mithridatic treatment
"STREPTOMYCIN / Weinstein and Ehrenkranz 27 HORMESIS Some organisms have been shown to be accelerated in their growth by streptomycin in vivo. Welch et al1°° found that the mortality rate of mice infected with Sal. typhosa was increased over that of the controls by small doses of streptomycin, whereas larger quantities afforded protection. This is the phenomenon of hormesis." -Streptomycin and dihydrostreptomycin . Weinstein, Louis, 1909-
Too much of a good thing
Meaning: Excess may do you harm.
Origin: The Oxford Dictionary of Quotations lists this phrase as proverbial and dates it from the late 15th century. The earliest example that I can find of it in print is from Shakespeare's As You Like It, 1600:
ROSALIND: Why then, can one desire too much of a good thing?
Come, sister, you shall be the priest and marry us.
Give me your hand, Orlando. What do you say, sister?