Sensitive or sensitised?
Two people describe an identical week. Everything stings, the moisturizer that was fine in spring now burns, and a bare face feels tight by lunchtime. One of them has been like this since childhood. The other was fine until she started a retinol in January. The complaints match exactly. The problems don't, and only one of them has an end date.
Sensitive is a trait
Sensitive skin reacts more readily than most to whatever touches it or surrounds it: friction, wind, cold, heat, pollution, a hormonal shift, a product that arrived last week. Redness, itching, stinging and burning are the usual vocabulary, and the pattern has generally been there for as long as you can remember.
It also keeps company with named conditions. Rosacea sits at this end, where blood vessels near the surface widen too readily and inflammation follows. So does eczema, where genetics play a part: some people make less filaggrin, the protein whose breakdown supplies much of the skin's own water-holding material, which leaves the skin barrier leakier than average from the start.
Sensitised is an event
Sensitised skin wasn't born reactive. It was made reactive, and almost always by something being done to it on purpose.
The routes are familiar. Exfoliating too often or too hard. Running several strong actives at once, where benzoyl peroxide plus a retinoid plus an acid pulls the barrier apart faster than it rebuilds, which in acne-prone skin drives oil production up and the acne with it. Foaming cleansers built on harsh surfactants, sodium lauryl sulfate being the clearest case. A toner or essence with denatured alcohol among its first few ingredients, which measurably raises water loss through the surface and lowers hydration.
The tell here is history again, running the other way. It didn't used to be like this. The same cleanser was unremarkable six months ago, and the moisturizer that stings today was fine all summer.
Why the answer changes the plan
Sensitised skin has a recovery window. Take the irritant away, run a plain routine, and mild disruption tends to ease inside one to two weeks. A more thorough job of it runs four to six, which is also roughly when products you used to get away with stop stinging again. There is an end to it, and your skin goes back to what it was before.
Sensitive skin doesn't have an end. What it has is a working range: a short routine kept permanently inside your tolerance, and a decent map of your own triggers. Nothing about that is a consolation prize, and plenty of reactive skin behaves beautifully once it stops being asked to cope with things it never liked. It just means you're managing a setting rather than repairing a dent, and you don't get to stop when it looks better. Most of this course is about drawing that map.
When it's both
This is the common case, and the one that trips people up. Naturally sensitive skin that has also been over-worked reads as a catastrophe, because both problems are talking at once and neither is easy to hear over the other.
Deal with the sensitised layer first, every time. You cannot take an accurate reading of your baseline through a flare. Strip back, let things settle for a few weeks, and only then start asking what your skin actually objects to. Go trigger-hunting while the barrier is still open and you'll convict ingredients that never did anything to you, then carry that list of bans around for years. Settle first. Investigate second.