Retinol is one of the few cosmetic ingredients backed by real scientific evidence: it smooths wrinkles, improves texture, evens out skin tone, prevents blackheads and reduces blemishes. But it comes at a price. Skin that isn't used to it needs a period of adjustment, and using it the wrong way brings irritation and unwanted reactions instead of a glow.

Pharmacist Marta Ortega has three reasons why autumn is the right moment. First, after summer the skin usually carries the consequences - dark spots, uneven texture and lost radiance - and retinol tackles exactly those problems. Second, autumn gives the skin time to adapt gradually before winter arrives. Third, retinol is the ingredient with the strongest scientific evidence against signs of sun damage.

Most bad experiences, says Ortega, come from a bad start. Her prescription is modest: two or three evenings a week, in a small amount, increasing the frequency as the skin tolerates it. Facial care specialist Ester Moreno warns that skin shouldn't "get used to it" through irritation - the process needs to be guided. The type of retinoid, the concentration and the frequency all have to be tailored to the individual skin.

This is where the cosmetics industry reaches for its favourite weapon - the number. A higher percentage sounds stronger on the label, but Alicia de la Lama-Noriega, co-founder of the brand R+, says formulation isn't a race to the maximum: "It's not about reaching the highest possible percentage, but about a concentration with a good balance between efficacy, stability and tolerability." The maximum permitted concentration in facial cosmetics is 0.3% retinol equivalent. The supporting ingredients matter too: niacinamide strengthens the skin barrier, and hydrating components improve comfort. "The active ingredient doesn't work on its own," she adds.

Moreno sums it up best: "Retinising your skin is like taking up running. Nobody who has never trained plans 20 kilometres on the first day." Next time a label promises you miracles with the strongest concentration on the market, ask yourself whether you need a marathon - or a bit of walking first.