How long can you keep a cosmetic product after opening it? (the PAO symbol)

A serum you started using months ago, a cream forgotten at the back of a drawer: Can you still use them? The answer is often found on the packaging, in the form of a small, discreet symbol that we don’t always notice. Knowing what it means lets you use your skincare products safely and get the most out of them.

Once opened, a cosmetic product has a limited shelf life, as indicated by the PAO symbol on its packaging. Here's how to read it, why cosmetics expire, and how to extend their shelf life.

The PAO symbol: What is it?

The PAO, which stands for Period After Opening, is represented by a small open jar symbol followed by a number and the letter M, such as 6M, 12M, or 24M. This number indicates the number of months during which the product remains at its best after it is first opened. A PAO of 12M therefore means that the cosmetic product should ideally be used within twelve months of opening.

Be careful not to confuse the two: this period begins when the product is opened, not when it is purchased. It is therefore a good idea to note the date you opened a product, especially for those you use only occasionally.

Best-by date or expiration date: What's the difference?

Two systems coexist. Products with a shelf life of more than thirty months display the PAO symbol—the well-known open jar. Those with a shelf life of less than thirty months display, instead, a best-by date, often preceded by the phrase “best used by” or symbolized by a small hourglass.

In summary: The best-before date provides information on how long the product will remain good after opening, while the expiration date indicates a time limit, regardless of whether the product has been opened or not. Both should be checked.

Why do cosmetics expire?

Two main factors come into play. The first is microbiological. With repeated use, the product is exposed to air, fingers, and applicators, which promotes contamination. Studies of cosmetics in use have shown that the vast majority of them harbor bacteria. Water-based formulas are particularly susceptible to this risk, which is why preservation systems are so important.

The second phenomenon is chemical: over time and with exposure to air and light, certain ingredients oxidize and lose their effectiveness. This is particularly true for sensitive active ingredients such as vitamin C. An expired product is therefore not just a matter of hygiene; it also loses its effectiveness, often accompanied by changes in color, odor, or texture.

How can you tell if a product has gone bad?

• The smell changes—it becomes sour, rancid, or unusual.
• The color changes—for example, a serum that turns yellow or brown.
• The texture changes: phase separation, thickening, or, conversely, liquefaction.
• The product becomes less effective, or it causes unusual discomfort.

If in doubt, it's best to avoid using the product, especially around the eyes or on sensitive skin.

How to Extend the Shelf Life of Your Cosmetics

• Follow the package insert and note the expiration date of products used occasionally.
• Close the containers tightly after each use to minimize contact with air.
• Store away from light, heat, and moisture, as the bathroom is not always the ideal place for sensitive products.
• Use clean hands or clean applicators to reduce the risk of contamination.
• Do not add water to a product, as this would promote microbial growth.

And what about PERS?

PERS skincare products feature a PAO symbol that indicates how long they can be used after opening. Certain active ingredients, such as the vitamin C in the PERS 10% Vitamin C Serum, are sensitive to oxidation: making sure to close the bottle tightly and store it away from light and heat helps preserve their effectiveness throughout the shelf life. Travel-size bottles are also useful for finishing a product under the right conditions, without letting it oxidize for too long.

Key Takeaways

Once opened, a cosmetic product has a limited shelf life, indicated by the PAO symbol (an open jar with a number of months). This period begins upon opening and should not be confused with the expiration date. Cosmetics spoil due to both microbial contamination and the oxidation of active ingredients, which affects their hygiene and effectiveness. By adhering to the PAO, closing products tightly, and storing them away from light and heat, you can extend their shelf life and fully enjoy their benefits.

Scientific Sources