The first of September is the day people come back from the sea and see themselves in daylight. The tan is good. What came with it - less so. Dark spots on the face affect around a third of women, and they appear even on those who diligently used sunscreen all summer.

The answer everyone knows is vitamin C. What few people know is that it matters which vitamin C.

What the pharmacist says

Vicente Calduch, a pharmacist and chief executive of the cosmetics company Calduch Especialidades, explains that vitamin C "works by regulating melanin production, which helps to gradually fade existing hyperpigmentation and makes it harder for new tone changes caused by sun or oxidative stress to form".

And he immediately adds: "Not all forms of vitamin C have the same effectiveness against hyperpigmentation."

The three forms, and which one you need

This is the detail that rarely makes it into the advertising. In a cream or a serum, vitamin C can be formulated in three ways.

The pure form - ascorbic acid. The most effective, but unstable and irritating, unless it is accompanied by other vitamins, most often niacinamide, that is vitamin B3.

A stabilised water-soluble derivative. Gentler, but with a more superficial action. It is recommended when the goal is glow or reducing acne scarring.

A stabilised fat-soluble derivative. This is the form Calduch recommends for dark spots. "Stabilised and fat-soluble versions retain their activity better and reach the skin better. That allows the treatment to be more consistent, and the results in evening out the tone more visible with regular use."

The reason is mechanical, not marketing. The fat-soluble form passes the skin's lipid barrier more easily, does not oxidise before it acts, and reaches the melanocytes - the cells that produce pigment - undamaged.

The part no product solves

"Consistency is the real treatment against dark spots," Calduch says. "Spots do not disappear overnight. Cell renewal takes time, which is why daily application is fundamental."

And the second, more important point: however powerful the serum, without sunscreen it is worthless. "No treatment against dark spots will be truly effective if the skin continues to be exposed to ultraviolet radiation without protection." Vitamin C is an antioxidant - it works against free radicals. It is not a shield.

So that means in September too, on a cloudy day too, in the rain too. Serum first, then sunscreen.

The industry sells you a new active ingredient every season. The two things that actually work - the right form of the vitamin and applying it every day - are not a new product, which is exactly why they get talked about the least.