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I believe that the concept is called Hormesis.

Kind of, but not precisely?

Hormesis is that idea that the same compound can have opposite effect at different doses.

But a dose difference is not strictly necessary for what I am talking about

Sill not what you're looking for, but another term to throw in the bag: "Iatrogenic"

A disease or problem which is a side-effects of medical investigation or treatment.

The term for the parent comment's concept is anti-fragile