PAO, Expiry Dates and Storage: How Long Cosmetics Actually Last

Most people know the little open jar exists. Far fewer know what it applies to, when it starts, or why some products carry a date instead. The result is a bathroom cupboard containing things bought optimistically in large sizes, half-used, and no longer quite what they were.

This guide explains the two dating systems, how packaging and storage change the answer, and how to buy sizes you will actually finish. It is about labels and storage, not skin: nothing here is medical advice.

The period-after-opening symbol explained

The open jar with a figure such as 6M, 12M or 24M is the period after opening: the number of months the manufacturer expects the product to remain suitable for use once opened.

Crucially, it does not appear on everything. It is required for products with a minimum durability of more than 30 months. Products expected to last 30 months or less carry a best-before indication instead — a date, sometimes accompanied by an hourglass symbol, or the words "best used before end of". So the absence of an open jar does not mean a product lasts indefinitely; it usually means the other system applies.

The clock starts at opening, because the period is about exposure — air, light, fingers, bathroom humidity — rather than shelf time. Two habits follow. Write the month you opened something on the packaging, since nobody remembers reliably. And treat the figure as the manufacturer's expectation under reasonable storage rather than a guarantee under any conditions.

Batch codes and how to check manufacture dates

Somewhere on the base, the crimp or the carton there will be a short alphanumeric string, printed or stamped rather than part of the artwork. That is the batch code, and its purpose is traceability: it lets the manufacturer identify which production run a specific unit came from.

It is not a consumer-facing date, and there is no standard format. Each manufacturer encodes its own information in its own way, which is why third-party batch decoder websites exist — and why they should be treated as unofficial. They can be a useful indication, particularly when buying from a marketplace or a discount reseller, but the authoritative answer comes from the brand or the retailer, who can be asked directly.

When batch codes matter most: buying deeply discounted stock, buying from an unfamiliar seller, or buying a product that has clearly been sitting. A brand that answers a batch query promptly is telling you something useful, in the same way that a specification published without prompting is, as discussed in how to judge product quality online.

Which formulas degrade fastest

Stability varies by formula type, and the pattern is reasonably predictable from the ingredients list.

Water-containing products — most creams, lotions, gels and liquid foundations — are the ones preservation systems exist for. Once opened and repeatedly touched, the preservative is doing continuous work.

Anhydrous products — oils, balms, sticks, powders — have no water for microbes and are generally more stable, though oils can oxidise and change smell over time.

Formulas built around fragile ingredients degrade on their own schedule regardless of microbial stability. Some actives are sensitive to air and light, which is why they arrive in opaque, airless or single-dose packaging — the formulation clue discussed in skincare actives and concentration.

Products applied near the eyes, particularly mascara, typically carry the shortest periods after opening because of how they are used: a wand repeatedly returned to a narrow tube. Follow the stated period rather than your own estimate here.

Packaging: pumps, jars, droppers and air exposure

Packaging is not presentation; it is part of the preservation system.

Airless pumps dispense without letting air back in, which protects both the preservative system and any air-sensitive ingredients. They are the most protective common format.

Standard pumps and tubes limit contact but admit some air. Good general-purpose protection with minimal fuss.

Droppers return the pipette to the bottle after touching skin, so contamination depends on technique. Avoiding contact with skin is the whole point of the design.

Open jars expose the entire contents at every use, and fingers introduce both microbes and oils. Perfectly reasonable for stable, well-preserved formulas; less good for fragile ones. Where you use a jar, a small spatula is a genuine improvement rather than a fussiness.

Sample sachets, decanted products and anything transferred into a different container fall outside the manufacturer's assumptions entirely, and no stated period applies to them.

Storage: heat, light and the bathroom problem

Most cosmetics are stored in the worst room in the house. A bathroom cycles through heat and humidity several times a day, and steam reaches inside imperfectly sealed packaging.

Three conditions do the damage. Heat accelerates chemical degradation and can separate emulsions. Light, particularly direct sunlight on a windowsill, degrades sensitive ingredients and is the reason opaque packaging exists. Humidity encourages microbial growth once a preservative system is under strain.

The practical fix costs nothing: store anything you care about in a bedroom drawer or a cupboard away from radiators and windows, keep lids properly closed, and do not leave products in a car or a sunlit bag. Refrigeration is unnecessary for most products and can cause separation in some, so follow the manufacturer's storage instructions rather than a general rule.

Signs a product has turned

• Separation that does not resolve, or a texture that has become grainy, watery or lumpy.
• Colour change — darkening, yellowing, or a shift in a pigmented product.
• Smell change, including a rancid or metallic note in an oil-rich product, or the loss of an expected scent.
• Behaviour change — a product that no longer emulsifies, absorbs or spreads as it did.
• Visible growth or anything unexpected on the surface.
• Packaging changes: a swollen container, a pump that has stopped working consistently, or a seal that has failed.

Any of these means stop using the product, regardless of what the label says or how much is left. If you experience a reaction, or you are unsure, a pharmacist is free to consult and is the right first stop — this guide cannot assess a specific product or a specific reaction.

Buying sizes you'll realistically finish

The largest size is frequently the worst value, because value is only realised if the product is used while it is still good.

Work it out before buying: estimate how much you use per application and how often you use it, and check that against the stated period after opening. A product with a short period, bought in a large size, used occasionally, will be discarded part-used — and the effective cost per application is far higher than the shelf price suggested. That arithmetic is set out in beauty cost per use.

Three practical rules. Buy the smaller size for anything you are trying for the first time. Buy the smaller size for anything with a short stated period. And be sceptical of multi-buy offers on products with short periods after opening, since the saving assumes a rate of use you may not achieve — the same discipline applied to sale pricing generally, and to the claims that accompany it in beauty marketing claims explained. Reading the ingredients list first, as in how to read a cosmetics ingredients list, tells you which category of stability you are dealing with before the size question even arises.