hormesis

What helps in a small dose can harm in a large one — and sometimes the very same substance does both. Hormesis describes a biphasic dose–response in which low doses produce a beneficial or stimulatory effect while higher doses of the same agent become inhibitory or toxic, tracing a U-shaped or inverted-U curve.

This contradicts the simple intuition that more is always more (or always worse). The proposed mechanism is overcompensation: a mild stress at low dose triggers protective, adaptive cellular responses, but as the dose climbs the stress overwhelms those defences and damage takes over. Classic illustrations include certain antioxidants, trace metals, radiation in some models, and physical exercise.

Hormesis is scientifically real in many systems but remains contested in how broadly it applies and how it should inform safety standards. It is a useful reminder that a dose–response curve is not always monotonic, and that extrapolating effects from high doses straight down to low doses — or vice versa — can be misleading.