Cosmetics carry a symbol indicating a period after opening, and most people either ignore it entirely or treat it as a hard expiry. Neither is quite right.
General information rather than advice about any specific product, and anything involving an eye infection or a skin reaction belongs with a doctor.
What the symbol actually means
An open jar with a number and the letter M indicates the manufacturer's assessment of how long the product remains suitable after it is first opened.
That assessment is based on stability and preservative efficacy testing, conducted under controlled conditions.
It is not a safety cliff. It is an estimate of the period during which the product will perform as intended and the preservative system will remain effective.
Separately, products with a shelf life under a stated period carry an actual expiry date, which is a different and firmer thing.
What actually determines deterioration
The variables that matter more than the number.
Water content, overwhelmingly. Anything containing water supports microbial growth and relies on a preservative system. Anhydrous products — powders, oil-based products, many lipsticks — are far more stable.
Packaging. A pump or a tube limits contamination. An open jar into which fingers go repeatedly introduces organisms continuously and depletes the preservative faster.
Storage. Heat and light degrade both the product and its preservatives, which is why a bathroom is among the worst places to keep cosmetics and is where everybody keeps them.
And use. A mascara wand pumped in and out draws air in each time, which is why mascara has the shortest recommended period of anything.
The categories that matter most
In descending order of how much attention they deserve.
Eye products, particularly mascara and liquid liner, where contamination has a direct route to a sensitive area and where infections do occur.
Anything in a jar containing water, where repeated finger contact and preservative depletion combine.
Sunscreen, where degradation reduces protection, and where the expiry date is a genuine functional matter rather than an aesthetic one.
And anything applied to broken skin or used after an infection, where the product should be discarded regardless of date.
What is considerably more durable than people think
Powders, which contain no water and are stable for years if kept dry.
Lipsticks, which are largely wax and oil and last a long time, though they can develop an off smell as oils oxidise.
Pencils, which are sharpened before each use and therefore present a fresh surface.
Which means a substantial proportion of a collection is fine well beyond the printed period, and the products that genuinely need replacing are a smaller set.
The signs worth noticing
More reliable than any date.
Change in smell, which is the earliest indicator for most products.
Separation that does not resolve on mixing.
Change in texture — thickening, becoming grainy, becoming watery.
Change in colour.
And any visible growth, which is unambiguous.
Any of these means discard regardless of what the packaging says.
The preservative debate
Worth addressing because it affects this directly.
Consumer pressure has driven reformulation away from certain preservatives toward alternatives.
The alternatives are frequently less effective across the range of organisms, which means shorter periods after opening and more reliance on packaging.
Preservative-free products exist and generally require refrigeration, single-use packaging, or a very short life.
Which is a genuine trade rather than an improvement, and it is rarely presented as one.
The practical routine
Write the opening date on the package, which takes two seconds and removes the guessing entirely.
Keep things out of the bathroom, or at least out of direct heat and light.
Replace eye products on schedule and be considerably more relaxed about powders.
Wash brushes, which are a larger contamination route than the products and which almost nobody cleans often enough.
And discard anything that smells different, which is the one test that has never misled me.
Brushes and sponges
Worth expanding since they are the larger contamination route.
Brushes accumulate product, oil and skin cells, and a brush used daily and never cleaned is transferring that back with every use.
Weekly washing with a mild soap addresses it, and drying flat rather than upright prevents water reaching the glue in the ferrule, which is what causes shedding.
Sponges are worse, since they are damp and porous, and they have been shown to harbour growth readily. They require frequent washing and periodic replacement regardless of appearance.
Sharing and testers
Two contamination routes worth naming.
Shared products, particularly eye products, transfer organisms directly between people and are a recognised route for eye infections.
In-store testers are used by many people and have been found to carry substantial contamination in testing.
Which is an argument for requesting a fresh sample rather than using the display unit, and for declining to share anything used near the eyes or on the lips.