Everything you need to know about stability testing for cosmetic products

In the process of developing a cosmetic product, there are crucial questions that need to be answered about the stability of a product on the long term, such as how long can a product last on the shelves while waiting for a customer to buy it, how to know if the container is compatible with the formula or under which conditions should the product be stored?

Those are questions that can only be answered by performing a stability test, which consists of subjecting a product to a series of tests for a set period to verify if it continues to meet all the characteristics that make it safe and suitable for sale.

But what are the requirements of a stability test? For how long does a product need to be tested? How can you interpret the results?

That is what we will be covering in this article together with clear guidelines in order to perform a valid stability test.

What is Stability Testing in cosmetics?

To begin with, a stable cosmetic product is a product capable of remaining the same, that is, maintaining the same characteristics, from its date of manufacturer for a determined period of time, without undergoing alterations or chemical reactions.

These alterations or chemical reactions can be detected by launching a stability testing program, in which samples of a cosmetic product will be stored under established temperature and relative humidity conditions. Certain characteristics of the cosmetic product will be evaluated over time, with the aim of verifying whether the product remains stable over the period of time suggested by the manufacturer (shelf life).

Stability testing will allow to know:

  • If a product is stable
  • If Interactions exist between ingredients, if the chosen ingredients are suitable and compatible with between them and not reacting with each other
  • If Interactions exist between the packaging and the formula, confirming if substances present in the packaging do not migrate to your formula and vice versa. The packaging also need to adequately protect the formula from light and the outside environment
  • Under which conditions the product should be stored to prevent unwanted changes to occur overtime
  • The shelf life by showing an estimation of how long your product will last on the shelves before being bought by a customer and still be suitable for use 

The shelf life granted to a cosmetic product is a particularly important piece of information as it determines whether to place an expiration date or a period after opening (PAO) on the product’s labelling.

What is a Compatibility Test?

A Compatibility Test is carried out with the aim of verifying the compatibility between the formula of a product and its final packaging (container) over a period of time, checking if the ingredients present in the product are compatible with the packaging materials. This test is  part of the Stability Testing of a cosmetic product.

Here are some phenomena that could occur during a compatibility test:

  • Leakage
  • Changes in product or packaging characteristics (appearance, color, odor, pH, etc.)
  • Changes in the packaging integrity
  • Signs of corrosion

To save time, it is recommended that the compatibility test be carried out in parallel of the stability test. To do so, the samples need to be tested while packaged in their final packaging, which needs to be monitored separately to make sure that that it does not break, leak, change appearance or change the appearance of the formula.

Which products need to be tested for stability and compatibility?

All cosmetic products must undergo stability tests as they ensure a product remains stable on the long-term. Furthermore, stability tests are an essential part of cosmetic Good Manufacturing Practices, which means they must be carried out during the product development stage. They are also necessary to prepare the Cosmetic Product Safety Report, required by the EU Cosmetics Regulation (EC) 1223/2009. It is one of the tests Safety Assessors review in order to determine a product’s safety. 

Finally, ensuring that a cosmetic product will remain safe and unchanged during its life cycle is key to gaining consumer loyalty and building a good reputation for your brand. In the end, there is nothing better than an effective, well-made and safe cosmetic product.

What are the requirements to perform a Stability/Compatibility Test?

First for all, a “proposed shelf life” for the product must be established. In other words, it is the period of time during which the product is expected to maintain its characteristics and to remain effective and safe for the consumer.

After giving a proposed shelf life, samples of the cosmetic product in the final packaging must be stored and incubated under different temperature and relative humidity conditions.

There are several types of Stability Tests, based on these conditions, but a product must undergo at least the following:

– Real-time or Long-term stability testing:

As its name suggests, this test is carried out in real time, that is to say for the entire duration of the proposed shelf life of the product. For example, if a shelf life of 2 years is assigned to the product, the real-time stability test should be performed for 2 years, to verify that the product is stable throughout this period. It is the most comprehensive and reliable test for determining the shelf life of a product.

A cosmetic product must be stable in all regions where it will be marketed. Therefore, long-term stability tests must be carried out taking into account the temperature and relative humidity conditions of the region where it will be marketed. As these tests must be carried out under normal conditions (ambient temperature), the climatic zones established by the ICH guidelines must be taken into account. For examples,  cosmetic products intended to be marketed in the European Union and the United Kingdom, climatic zone II is generally considered (25 ºC and 60% relative humidity).

Climatic ZoneType of ClimateTemperatureRelative Humidity
ITemperate21ºC ± 2ºC45% ± 5%
IIMediterranean / Subtropical25ºC ± 2ºC60% ± 5%
IIIHot and Dry30ºC ± 2ºC35% ± 5%
IV/aHot and Humid / Tropical30ºC ± 2ºC65% ± 5%
IV/bHot / Higher Humidity30ºC ± 2ºC75% ± 5%

– Accelerated stability testing:

This is a test carried out under high temperature conditions, with the aim of accelerating the chemical reactions that can degrade the cosmetic product and predict how it will behave in the long term. Because the degradation process is accelerated, it is not necessary to carry out the test during the entire proposed shelf life. It is recommended to carry out the test for a minimum period of 3 months. For cosmetic products to be marketed in the European Union and the United Kingdom, these tests are usually carried out at 40-45ºC and 75% relative humidity.

Keep in mind that an accelerated stability test must always be backed up by a long term stability test. 

– Reduced temperature testing:

This is a test carried out at low temperatures in order to have a “blank”, that is, a reference product with which to compare samples stored under long term and accelerated conditions. Typically, samples stored at low temperatures will undergo changes at a very slow rate so you can take these as references. In addition, this test can give additional data on the behavior of your cosmetic product at low temperatures. These tests should last as long as the long term stability test.

Which tests for which products?

Depending on the type of product certain tests will be advised rather than others. Feel free to reach out to Taobé so we can advise on the tests to be performed according to the type of your products. 

In general, the parameters to check are the same as the ones checked to release a product for sale (the ones detailed on the product’s technical data sheet or certificate of analysis), adding some additional parameters to check the product’s compatibility with the packaging (% weight loss, appearance and integrity, visible leakage, etc.).

How to calculate the Shelf Life?

It is known that accelerated stability tests normally increase the degradation process of a product by 2 to 3 times, for every 10ºC increase in temperature.

Therefore, if it is determined that a cosmetic product is stable for 12 weeks (3 months), at a temperature of 40ºC-45ºC, it could be concluded that the product will be stable for at least 24 months (2 years). This conclusion must always be supported by the result of a long-term stability test.

Which parameters need to appear on my Stability Test Report?

It is recommended that the stability test report be as complete as possible, so many parameters can appear on the test, but the report must contain at least the following information:

  • Name of the Cosmetic Product
  • Batch tested
  • Test start date
  • Packaging description and materials
  • Type of Stability Test and testing conditions (temperature and relative humidity)
  • Scheme of tests carried out for the established period, detailing the specification that must be met.
  • Results
  • Conclusion, which must include the shelf life granted to the product, based on the results

Do Stability Tests need to be performed by an independent lab or can it be done in-house?

Stability tests can be perform in-house if you have a lab will all the necessary equipment to carry out each of the tests. If you don’t have a well-equipped lab, you can always outsource stability testing.


Another information that stability tests will reveal is the shelf life of the product. The shelf life is the time during which a product can remain on a shelf and still be good for consumption.

In order to determine which symbol has to appear on a product, the rule is simple:

  • If the shelf life is less than 30 months : your product needs to show the hourglass symbol + Best before + expiry date
  • If the shelf life is greater than 30 months : your product needs to show the PAO symbol with a number of months inside the symbol

Once the symbol has been determined, the period of time during which the product remains good for consumption after it has been opened has to be calculated:

  • If the hourglass symbol must be used, the expiry date should be printed on the packaging at the time of production and it must be calculated based on the shelf life granted to the product after stability testing was carried out on the product. If the real time stability test determined a shelf life of 6 months, the expiry date could be of 6 months maximum, starting from the production date.
  • If the PAO symbol must be used, the number of months to be written inside the little jar symbol is usually determined by the manufacturer, which calculates it based on the stability test results + the guidelines as recommended by the Afssaps.

Do you still have questions about the stability tests?

At Taobé we can give you advice on every detail of your stability testing program. So, if you have any questions or need help, do not hesitate to contact us!


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