Actives

'Active' is an informal term for an ingredient included to produce a measurable change in the skin, rather than to make a product feel or smell pleasant.

The usual ones: retinoids, exfoliating acids (AHA, BHA, PHA), vitamin C, benzoyl peroxide, azelaic acid, and — at the gentle end — niacinamide.

Two things follow from that definition. First, actives take time: four to twelve weeks depending on what you are treating, because they work with the skin's own cycle rather than overriding it. Second, they can irritate, and the irritation is usually a function of frequency rather than strength.

The practical rules are the same for all of them:

- Introduce one at a time, two weeks apart. Three at once means no information about which worked and which caused the problem. - Start at a low frequency and build, not the other way round. - Never layer two strong actives in one application. - Wear sunscreen daily. Every active on that list makes sun protection more important, not less.

A routine does not need more than one active to work. Most people's results come from consistency, not from the number of bottles.