Is there a word for a cure which is also a poison?

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.

Solution 1:

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.)

Solution 2:

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.