Hormesis is a bi-phasic distribution that describes the potential benefit of certain stimulii in low-doses that may elicit a negative effect at larger doses. For example, low-doses of light radiation may signal neuroprotective responses on an intracellular level but, at higher levels of exposure, might cause neuronal damage.
The area in which a favorable response is caused by low-level exposure is called the hormetic zone. This modeling of dose-dependent toxicity isn’t rigidly agreed upon by scientists.
This phenomenon was first observed by Hugo Schultz following increased growth rates of yeast following low-dose exposure to toxins. Later work by German physician Rudolph Arndt complemented Schultz’s and eventually gave way to the following mantra:
For every substance, small doses stimulate, moderate doses inhibit, large doses kill.
Certain other, non-pharmacological, therapies such as cold-water immersion therapy, intermittent hypoxic training, or even infrared light therapy all illustrate this principle at work. That does not translate to universal applicability by any means.