Vitamin C is the most bought and most wasted active in skincare – wasted because a great many bottles oxidise on a shelf before they're finished. Understanding which form you're buying, and how to store it, matters more here than with any other ingredient.
The forms, and why they differ
L-ascorbic acid is pure vitamin C, the form with the most research behind it and the one your skin can use directly. It's also notoriously unstable: exposure to light, air and heat degrades it. A serum that has turned deep orange or brown has oxidised, and while it isn't harmful, it's no longer doing much.
Derivatives — sodium ascorbyl phosphate, magnesium ascorbyl phosphate, ascorbyl glucoside, tetrahexyldecyl ascorbate and others — are more stable and gentler, but need converting by the skin, so they generally work more slowly.
Neither is simply better. If your skin is robust and you'll use the bottle within a few months, L-ascorbic acid is the more direct route. If your skin is sensitive, or you want something that stays usable, a derivative is the more practical choice.
What it's used for
Vitamin C is an antioxidant, and in cosmetic terms it's chosen for the appearance of dullness and uneven tone. Skin typically looks brighter and more even with consistent use. It's the ingredient most associated with radiance, and it's the reason so many people use it in the morning.
Concentration and packaging
L-ascorbic acid formulas usually run between 10% and 20%. Below about 8% there's little point; above 20% there's little gain and more chance of irritation. Fifteen percent is a sensible target for most people.
Packaging tells you a lot about a formula. Opaque or amber glass, an airless pump, a small bottle — these suggest a brand that understands the stability problem. A clear jar suggests one that doesn't. Store it away from the bathroom window, and don't buy a year's supply at once.
How to use it
Morning, after cleansing, before moisturiser and sunscreen. Vitamin C and SPF are natural partners — the antioxidant complements the protection, and this is the routine most dermatologists would recognise.
It pairs particularly well with vitamin E and ferulic acid; that trio appears together in many formulations because the combination is more stable than vitamin C alone.
Using it alongside retinol is straightforward: vitamin C in the morning, retinol at night. And despite what you may have read, niacinamide and vitamin C can be used together — that warning comes from decades-old research on raw ingredients, not modern formulas.
If it stings
L-ascorbic acid is formulated at a low pH, which is why it can tingle on application. A brief tingle is normal; persistent stinging is not. Try a lower percentage, a derivative form, or applying to fully dry skin after cleansing.
Explore our vitamin C serums and moisturisers, or see all active skincare.