No observed effect level (NOEL)

Highest dose of a drug that has no observable impact distinguishable from those seen in controls.

NOEL refers to any effect, but is commonly applied to adverse effects, in which case it is synonymous with NOAEL.

Guidance for Industry Estimating the Maximum Safe Starting Dose in Initial Clinical Trials for Therapeutics in Adult Healthy Volunteers (FDA, 2005)

REVIVE Webinar: Making Safety a Part of Drug Design (GARDP, 2018)