Does the cooling mean it is working?

It means a cold receptor was stimulated. That is a sensation rather than an effect on skin, and it happens just as reliably in a product with nothing else in it.

Does menthol shrink pores or help acne?

No. The tight, fresh feeling is the sensation plus a little evaporation. Pore size is not changed by it, and nothing here treats acne.

Why does my toner tingle?

Usually menthol, alcohol or an acid, or several at once. A tingle is not a measure of strength, and when it is the cooling agent it is telling you nothing about the rest of the formula.

Is it safe for sensitive skin?

It is one of the first things to remove if skin is reactive. Not because it is dangerous, but because it adds a nerve stimulus without adding a benefit, and it hides the feedback you would use to judge everything else.

Is natural menthol different from synthetic?

Not to skin. It is the same molecule whether it came from mint oil or from a reactor, and the INCI name does not separate them.

Menthol — Ingredient Guide

How it works

Menthol is an alcohol found in mint oils and also made synthetically. The INCI name covers both and the label does not distinguish them. Its effect is not a change in temperature but a message: it binds a cold-sensing receptor on sensory nerve endings in the skin, the same one that responds to an actual drop in temperature, and the nerve reports cold to the brain while the skin's temperature has barely moved.

Nothing structural happens. No barrier is repaired, no oil is removed and no inflammation is reduced by the cooling itself. It also smells of mint, so in many products it doubles as part of the scent, and that is the whole of its contribution.

The evidence

The sensation is real, well characterised and reliably reproducible. What it is evidence of is where the care is needed. Cooling and soothing feel adjacent and are not the same thing: a product that makes an angry patch feel better for ten minutes has changed how it feels rather than what is happening in it, and the relief wearing off is not a sign the product stopped working, because there was nothing to stop.

The same receptor stimulation, pushed harder, stops reading as pleasant and reads as stinging or burning, and the level at which it flips is lower on skin whose barrier is already compromised. There is a second-order problem worth naming. A strong sensation is the most persuasive thing a formula can offer, and it can cover the sting that would otherwise have told you a product was too much for you. Nothing here treats a skin condition.

Suitability

Not an active; no skin type is better served by its presence
Reactive and flushing-prone skin has the clearest reason to avoid it
So does eczema-prone, shaved, sunburnt or freshly exfoliated skin
Common in cleansers, toners, aftershave products and eye products sold on a fresh feeling

Concentration

No figure, and the useful thing is to say why. The gap between pleasantly cool and unpleasantly burning is individual and moves with the state of the skin, so a level one person finds refreshing is another's sting on the same day. What a market permits in a cosmetic is set locally and gets revised, and a given product's level is not something the list reveals.

Conflicts & combinations

No chemical incompatibility. The scheduling matters more. Over an acid, a retinoid or an alcohol-heavy toner, menthol turns a tolerable step into a stinging one, and that is a common reason people conclude an active does not suit them when the cooling agent was the problem. If a product stings, the useful response is to work out which of the two did it rather than to keep both.