No-observed-adverse-effect level

The no observed adverse effect level (NOAEL) denotes the level of exposure of an organism, found by experiment or observation, at which there is no biologically or statistically significant (e.g. alteration of morphology, functional capacity, growth, development or life span) increase in the frequency or severity of any adverse effects in the exposed population when compared to its appropriate control.

In toxicology it is specifically the highest tested dose or concentration of a substance (i.e. a drug or chemical) or agent (e.g. radiation), at which no such adverse effect is found in exposed test organisms where higher doses or concentrations resulted in an adverse effect.

This level may be used in the process of establishing a dose-response relationship, a fundamental step in most risk assessment methodologies.

In drug development, NOAEL of a new drug is assessed in laboratory animals drugs prior to initiation of clinical trials to establish a safe clinical starting dose in human trials.