threshold dose

Below a certain amount, a drug does nothing you can detect — like adding a single grain of salt to a pot of soup. The threshold dose is the smallest dose that first produces a just-detectable effect; anything below it leaves the response indistinguishable from baseline.

On a dose–response curve the threshold is the foot of the curve, where the line just lifts off the floor. Its existence reflects the buffering capacity of biological systems: a few occupied receptors or a small perturbation can be absorbed without any measurable change until enough of the system is engaged. The threshold is therefore tied to the sensitivity of the assay and the endpoint chosen.

Thresholds matter most in toxicology and risk assessment, where a key debate is whether a given harm has a threshold below which it is safe, or is non-threshold (any dose carries some risk, as is often assumed for genotoxic carcinogens). For therapeutic effects the threshold dose is the conceptual cousin of the minimum effective concentration measured in plasma.