Here’s a fact that surprises many people outside the industry: there are currently no specific US requirements for conducting stability testing or assigning expiration dates to cosmetic products. A brand could, in theory, sell a product indefinitely without ever studying how it changes over time. So why do so many cosmetic and personal care companies invest in stability testing anyway?
The answer comes down to two practical realities. First, US law holds cosmetic product owners responsible for the safety of their products for as long as those products remain available for sale and use. Without an expiration date on the label, that responsibility never has a natural endpoint. At some point, time affects the quality and potentially the safety of nearly every product, and an expiration date gives the manufacturer a defined boundary for that responsibility. Second, stability testing simply makes good business sense. Confirming that a preservative system stays effective and that quality attributes hold up over time reduces the likelihood of consumer complaints, adverse events, product recalls, and lost market share.
A stability study is, at its core, a program of testing designed to show how a product’s quality changes over time under various environmental conditions, such as temperature, humidity, and light. Its primary purpose is to establish either a shelf life (expiration date) or recommended storage conditions for the product in the container system in which it’s actually sold.
There are three common types of stability studies, each answering a different question:
RT Studies
Real-time studies, conducted at 25°C and 60% relative humidity, demonstrate actual shelf life and support real-time expiration dating.
Accelerated Studies
Accelerated studies, conducted at 40°C and 75% relative humidity, use exaggerated environmental conditions to extrapolate expiration dating more quickly than a real-time study would allow.
Cold Studies
Cold (refrigerator) studies, conducted at 5°C, provide insight into how a product behaves after exposure to cold conditions, which matters for shipping, storage, and certain climates.
Storage conditions and testing timepoints for these studies are typically based on ICH guidelines, commonly including 25°C/60% relative humidity checkpoints at initial, 3, 6, 12, 24, and 36 months, and 40°C/75% relative humidity checkpoints at initial, 1, 3, and 6 months, with an intermediate condition of 30°C/65% relative humidity sometimes used as a backup.
At each of these timepoints, a range of tests is typically conducted: physical appearance and odor, evaluation of any interaction between the product and its primary container, antimicrobial effectiveness testing, pH, viscosity, specific gravity, microbial limits testing, and chemical assay of preservatives or functional ingredients, along with any other tests appropriate to the specific product type and delivery system.
Stability testing isn’t a regulatory obligation for US cosmetics, but it functions as a form of due diligence that protects consumers, supports label claims, and gives a brand real evidence that its product performs as promised, from the day it’s manufactured to the day a consumer finally opens the container.