Usage Examples
- The 28-day rat study set the NOAEL at 30 mg/kg/day on histopathology and clinical chemistry.
- Module 2.4 has to explain why the dog NOAEL, not the rat NOAEL, drove the starting dose.
- The reviewer disputed our NOAEL and asked us to justify why the enzyme elevation was non-adverse.
What is No Observed Adverse Effect Level (NOAEL)?
No Observed Adverse Effect Level is the highest dose in a nonclinical toxicology study producing no biologically significant adverse finding relative to controls, and it anchors the human starting dose for a first-in-human trial.
No Observed Adverse Effect Level exists because the first dose given to a human being has to be justified from something other than human data. Animal toxicology produces a graded dose-response, and the NOAEL is the point on that curve below which nothing harmful was seen. It converts a toxicology dataset into a single defensible number that a reviewer can check and challenge.
No Observed Adverse Effect Level covers findings in treated animals measured against concurrent controls across the study's endpoint set: clinical signs, body weight, food consumption, clinical pathology, organ weights, and histopathology. Its boundary is narrow. The NOAEL is not a human safety threshold, not evidence that any dose is safe, and says nothing about the untested gap between dose groups, longer dosing, or a different species.
No Observed Adverse Effect Level is applied by carrying the value from the most appropriate species into the human starting-dose calculation, then documenting the species choice and the adversity calls in Modules 2.4 and 2.6 of the eCTD. The arithmetic is not the hard part. Defending which findings were adverse is, because a contested NOAEL moves the starting dose down.
Not to be confused with
- NOEL
- NOEL is the highest dose producing no observed effect of any kind, including pharmacologic changes that are not harmful. NOAEL permits those non-adverse effects, so for the same study NOAEL is equal to or higher than NOEL.
- LOAEL
- LOAEL is the lowest dose that did produce an adverse effect, which in practice is the next dose group above the NOAEL. The real threshold sits somewhere in the untested interval between the two, which is why dose spacing determines how precise your NOAEL is.
- MTD
- the MTD is a high-dose ceiling used in study design; ICH M3(R2) Section 1.5 treats it as one option for characterizing potentially clinically relevant effects, alongside exposure saturation and the maximum feasible dose. The NOAEL sits below it and is an outcome of the study, not a design input.
- MABEL
- MABEL is anchored on the minimum anticipated biological effect level derived from pharmacology data, not on toxicology findings. It is a different input with different logic, not a more conservative NOAEL.
The NOAEL itself is not a regulation. These are the obligations that govern the studies that produce it and the submission that relies on it.
What you must do
- 1Determine the NOAEL in nonclinical safety studies performed in the most appropriate animal species, and treat it as the most important information for estimating the first dose in humansICH M3(R2) Section 6
- 2Weigh the pharmacological dose response, the pharmacological and toxicological profile, and pharmacokinetics alongside the NOAEL rather than deriving the starting dose from the NOAEL aloneICH M3(R2) Section 6
- 3Support a clinical development trial of up to 2 weeks with repeated-dose toxicity studies in two species, one non-rodent, of at least 2 weeks' duration, and match longer trials with at least equivalent study durationICH M3(R2) Section 5.1
- 4Select the high dose so that potentially clinically relevant effects are adequately characterized, using the MTD or an equally appropriate limiting dose such as exposure saturation or the maximum feasible doseICH M3(R2) Section 1.5
- 5Submit the pharmacology and toxicology information underlying the NOAEL in the IND as the basis on which the sponsor concluded it is reasonably safe to conduct the proposed clinical investigations, and state for each study subject to Part 58 whether it complied with GLP or why it did not21 CFR 312.23(a)(8)
- 6Run the safety studies that generate the NOAEL under good laboratory practices, since Part 58 governs nonclinical laboratory studies intended to support applications for research or marketing permits21 CFR 58.1
Common mistakes
Deciding adversity on statistical significance alone
A consistent, dose-related shift can be adverse without reaching p<0.05 at n=10, and a meaningless fluctuation can reach it. A NOAEL argued purely on p-values is the one reviewers send back, and the correction arrives as a lower starting dose after the IND clock has already started.
Reading the NOAEL as a safe dose
The NOAEL is one tested dose, in one species, on a defined endpoint set, for a defined duration. It demonstrates nothing about intermediate doses, longer exposure, or humans. Treating it as proof of safety is how sponsors over-reach on the starting dose and lose the argument in review.
Accepting a NOAEL from a study whose high dose was too low
If the top dose produced no effect, you have a NOAEL but no toxicity characterization and no upper bound, which is exactly what ICH M3(R2) Section 1.5 is written to prevent. The remedy is a repeat study, which costs a full nonclinical cycle before the IND can move.
When This Matters
- The 28-day rat study set the NOAEL at 30 mg/kg/day on histopathology and clinical chemistry.
- Module 2.4 has to explain why the dog NOAEL, not the rat NOAEL, drove the starting dose.
- The reviewer disputed our NOAEL and asked us to justify why the enzyme elevation was non-adverse.
Frequently Asked Questions
NOAEL is the highest dose with no adverse effect, while NOEL is the highest dose with no observed effect of any kind, adverse or not. NOEL is therefore equal to or lower than NOAEL, because a pharmacologic change that is not harmful breaks the NOEL but not the NOAEL.
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