How to find out what your skin reacts to

Most people never find out what their skin reacts to. Not because it is impossible, but because the way we normally shop makes it impossible to tell. Three new things arrive in the same week, something goes wrong, and there is no way to know which one did it.

This is a method for finding out. It takes patience rather than expertise, and it costs nothing.

What this is not. It is not a way to diagnose yourself, and it is not medical advice. If your skin is painful, broken, bleeding, spreading, or not settling, stop and see a doctor. Nothing here replaces that. If you are using something a doctor prescribed, do not stop it because of anything written here.

Before you start: three rules

One. Change one thing at a time. This is the whole method. Everything else is detail.

Two. Give it long enough. A reaction is not always immediate. See the timing section below for why two weeks is the right number and not a cautious one.

Three. Write it down. Not because you will forget that something happened, but because you will forget exactly when. The timing is the part that tells you what kind of reaction you had, and it is the part memory loses first.

The method

Step 1. Go back to a quiet baseline.

For one week, use as little as possible. A gentle cleanser and one moisturiser you have used for a long time without trouble. Nothing new, nothing active, nothing you are unsure about.

You are not treating anything during this week. You are establishing what your skin is like when nothing is being added to it. Without that, you have nothing to compare against.

Step 2. Test the new product on a small area first.

Choose somewhere you can watch easily and that is not your whole face. The inner forearm, or the skin just in front of the ear along the jaw, are both reasonable.

Apply a small amount, once a day, to the same spot. Leave everything else as it was in step 1.

Step 3. Watch for five days.

Look at the spot at roughly the same time each day, in daylight if you can. Write down the date, the time you applied it, and what you see. If you see nothing, write that down too. A week of "nothing" is a result.

Step 4. If nothing happens, move to the face.

Use the product normally, still changing nothing else. Keep the log going.

Step 5. Give it two weeks before you judge it.

Two weeks for two separate reasons. It is long enough for a delayed reaction to appear, and it is long enough that you are not judging a product on how it felt in the first thirty seconds, which is the easiest thing in skincare to engineer and the least informative.

Step 6. Only then add the next thing.

Then repeat from step 2. Yes, this is slow. It is also the only version that produces an answer.

The log

Print this, or copy it into your notes. The columns matter more than the format.

A filled example:

That log is worth more than a paragraph of description, because the last column tells you something the description cannot.

How to read your log

This is the part almost nobody knows, and it is the reason the log exists.

Two different things get called "a reaction", and the difference is time.

Irritation is fast. It is direct injury: a substance damages cells in the upper layer of skin, and the body responds with inflammation. It typically appears within minutes to hours, and often announces itself as stinging or burning while the product is still on. It is a matter of dose and exposure, which is why the same substance in a smaller amount, or on less compromised skin, may do nothing at all. The clinical name is irritant contact dermatitis.

Allergy is slow. The substance is not injuring you. Your immune system has learned to recognise it and now responds every time it appears. This is a delayed type IV hypersensitivity reaction, and it typically appears 48 to 72 hours after exposure, because the immune response has to be mobilised before anything becomes visible. The clinical name is allergic contact dermatitis.

So:

In the example log above, the reaction appeared at 72 hours. That pattern is consistent with an allergy, and it is exactly the pattern you would miss if you had started three products on the Monday.

Why this matters practically. Irritation is about exposure, so using less, less often, or on a smaller area can be enough, and skin that has calmed down may tolerate the same product later. Allergy is about recognition, and immune memory does not fade to order. Once the immune system has learned a substance, it tends to keep knowing it.

One thing to look at on the label

You do not need to understand an ingredients list to get something out of it.

The order is the information. Ingredients are listed in descending order of weight down to one per cent. Below one per cent the order is free. So the first five entries are most of what you are actually holding. Everything after them is present in small amounts.

And one word is worth knowing. "Parfum" or "fragrance" is a single word that can stand for dozens of substances. The composition does not have to be broken down on the label. So if your skin reacts to a product containing it, there is no way to work out which part it reacted to. That is a real limitation of the label, not a claim about any product.

What this method cannot do

It cannot tell you which of the two you have. It narrows the field, and that is all it does.

Patch testing by a dermatologist is how an allergy is actually identified. It exists precisely because guessing is unreliable even for experts, and a method sheet is a form of guessing. What your log does is hand a doctor something specific instead of "I think something is irritating my skin", and that is genuinely worth having.

If your skin is painful, broken, or not settling, that is a question for a doctor rather than for a moisturiser.