no-observed-adverse-effect level

The no-observed-adverse-effect level, or NOAEL, is the highest dose of a drug at which animals in a toxicology study show no harmful effect that can be detected. Picture turning up a dial of dose, step by step, and noting the last setting before any sign of damage appears; that setting is the NOAEL.

It is established in carefully controlled animal studies that monitor body weight, blood chemistry, organ histology, and clinical signs across several dose groups. The lowest dose that does cause an adverse effect is the LOAEL; the NOAEL sits just one dose step below it. Note that 'no observed' depends on what the study looked for and how sensitive it was, so the NOAEL is partly a function of study design.

The NOAEL is a cornerstone of safety assessment because it anchors the starting and maximum doses chosen for first human trials. Regulators take the NOAEL, convert it to a human-equivalent dose, and apply safety factors to set a cautious first-in-human dose. A higher NOAEL relative to the effective dose translates into a wider safety margin.