Serums

A serum is a delivery format rather than a category of ingredient: lighter than a cream, more concentrated, and designed to sit under a moisturiser rather than replace it.

That is what makes it the usual home for actives such as vitamin C, retinol, niacinamide and hyaluronic acid. Applied to slightly damp skin and sealed with a moisturiser, a serum does more than the same active in a heavier base.