How to tell when a product has stopped working

Three very different things get described the same way: it stopped working.

A product that worked for a year suddenly does not. The instinct is to assume it has changed, replace it, and move on. Sometimes that is right. Often the product is fine and something else has changed — and buying a replacement makes the picture harder to read rather than easier.

There are three distinct situations here and they need different responses.

One: the product has genuinely degraded

Cosmetics have a shelf life, and it starts from opening rather than purchase. The small open-jar symbol on the packaging carries a number — 6M, 12M, 24M — which is the period after opening the manufacturer expects the formula to hold.

The reliable signs a product is finished, regardless of what the date says:

• The colour has changed. A vitamin C serum turning orange or brown is the classic example, and it means potency has gone.
• The texture has separated or gone grainy, watery, or unusually thick.
• The smell is different. A faintly rancid or sour note in anything oil-based means the oils have oxidised.
• It stings when it did not before. This is the one people ignore, and it is the clearest signal there is.

Storage accelerates all of this. A bathroom shelf is warm and humid; a car, a bag and a windowsill are worse. A cool dark cupboard genuinely extends the life of everything in it.

Two: something else changed, and the product is fine

Skin is not a fixed thing. It responds to season, indoor heating, air conditioning, water hardness, sleep, stress, medication and age. A moisturiser that was right in July is frequently too light in January, and that is not the moisturiser failing.

The other common invisible change is water. Moving house, or a change in the local supply, can alter how every product in the shower behaves. If shampoo suddenly seems to stop lathering, hair feels coated, and soap leaves a film on glassware, hard water is the likely explanation and no new shampoo will fix it — a chelating shampoo used occasionally will.

Habit drifts too. Amounts get smaller as a bottle gets more precious. Application gets faster. Two steps quietly become one. It is worth genuinely checking whether you are using a product the way you were six months ago before concluding it has changed.

Three: it never worked, and you are only noticing now

The uncomfortable one. A product bought in a good mood, used inconsistently, and credited with results that came from something else in the routine. This is why introducing one product at a time matters so much: it is the only way to attribute a result to a cause.

If you cannot say what a product changed, when it changed, and what you would look for to know it was working, it may simply never have been doing anything.

The eight-week rule

Most personal care products need six to twelve weeks of consistent use before a fair judgement is possible. Skin cell turnover takes roughly a month; hair grows about a centimetre in the same time. Anything assessed at two weeks is being assessed on the placebo, the weather and the lighting.

The exceptions are irritation and allergy, which are immediate or near-immediate. Burning, stinging, spreading redness, swelling or itching that does not settle are not a product "adjusting" — they are a reason to stop.

Purging versus breaking out

A genuine adjustment period exists for a narrow set of products that speed up cell turnover, where breakouts may briefly appear in places you normally break out, and settle within four to six weeks. That is the actual, limited claim.

It gets stretched to cover any product causing any reaction, which is how people persist for months with something that is straightforwardly irritating them. Two useful distinctions: purging happens where you usually break out, not in new places, and it improves rather than escalating. If it is spreading, worsening, or somewhere you never break out, it is not an adjustment period.

What to do, in order

1. Check the product first. Colour, texture, smell, sting, and the open date you wrote on the bottle.
2. Check what else changed. Season, heating, water, medication, sleep, a new product somewhere else in the routine.
3. Check your own application. Amount, frequency, and whether a step quietly disappeared.
4. Change one thing. Not three. One, and then wait.
5. Give it eight weeks unless it is irritating you, in which case give it none.

How to test a suspicion properly

Most people diagnose this by changing several things and seeing what happens, which produces no usable information. A cleaner test takes a fortnight.

1. Stop the suspect product only. Keep everything else identical, including the amounts and the timing.
2. Wait two weeks. Long enough for irritation to settle, short enough to stay patient.
3. If things improve, reintroduce it once. If the problem returns, you have your answer. If it does not, the product was not the cause.
4. If nothing changed, put it back and suspect something else — the season, the water, a different product, or your own application.

This is slower than replacing the bottle. It is also the difference between knowing and guessing, and guessing is what turns one unhelpful product into a cupboard of them.

The three culprits people rarely check

Amount. As a product becomes precious, the amount applied shrinks — usually unconsciously. Sunscreen is the extreme case, but it happens with everything expensive. Measure it once against what you were using when it was new.

Water. Hard water changes how every product in the shower behaves: shampoo lathers less, hair feels coated, soap leaves film on glassware. It can change after a house move, and no amount of switching products addresses it. A chelating shampoo every few weeks does.

Something else in the routine. A new product added six weeks ago can make an existing one appear to stop working — a new exfoliant leaving skin more sensitive, a new styling product building up on hair. The product that changed is not always the product that is failing.

What to do with something that has genuinely turned

Throw it away. Not "use it up on my body", not "keep it for emergencies". A product whose colour, texture or smell has changed has changed chemically, and using it on a larger surface area is a worse idea rather than a thriftier one.

The two categories where this is not a judgement call: sunscreen, where degraded filters mean you are protected less than the label says and you cannot see it happening; and anything used near the eyes, where the risk is contamination rather than performance. Both go in the bin at the date.

When it is not the product at all

If skin or hair is changing in a way that a routine adjustment does not explain — persistent breakouts that will not settle, a rash, itching without a cause, flaking that keeps coming back, hair shedding noticeably more than usual, or any change in a mole — that is the point to see a professional rather than to keep changing bottles.

Nothing on this page, and nothing in a bathroom cabinet, is a diagnosis. A product that has stopped working is a small problem. Something that keeps getting worse regardless of what you use is a different kind of problem, and the useful move there is an appointment rather than a purchase.

General cosmetic information, not medical advice and not a diagnosis. Skin, hair and circumstances differ — patch test anything new, and see a professional for anything persistent, painful, spreading or worsening.