Vitamin C in skincare — what it does, what it cannot do, and why form matters.

Vitamin C is one of the most studied topical actives in dermatology. It is also one of the most frequently misrepresented by brands. The gap between what vitamin C can do and what it is marketed to do is large enough to matter.

What vitamin C is

Vitamin C (ascorbic acid and its derivatives) is a water-soluble antioxidant and a co-factor in collagen synthesis. In the skin, it serves two primary functions: it neutralises reactive oxygen species before they damage cellular structures, and it participates directly in the biochemical pathway that produces collagen.

These two functions have real, measurable effects on skin over time: reduced oxidative damage from UV exposure, improved skin tone (particularly post-inflammatory hyperpigmentation), and — with consistent long-term use — measurable improvement in collagen density. This is not marketing. It is documented in controlled trials.

What vitamin C cannot do

It cannot replace SPF. Vitamin C reduces oxidative damage from UV radiation, but it does not block UV. The common framing of "antioxidant protection" implies photoprotection it does not provide. SPF is non-negotiable and separate.

It cannot reverse deep structural changes to the skin. Vitamin C supports collagen synthesis; it cannot undo decades of existing degradation in a meaningful timeframe. The clinical evidence for improvement in fine lines is real but modest, and the timeline is months, not weeks.

It cannot absorb through the skin if the formulation is wrong. This is the most practically important point.

Why formulation determines efficacy

L-ascorbic acid (LAA) — pure vitamin C — is the gold standard. It has the most clinical evidence. It is also inherently unstable: it oxidises rapidly when exposed to air, light, or heat, turning yellow and then brown. Oxidised vitamin C does not produce the same antioxidant benefits; it may also cause irritation.

For LAA to penetrate the stratum corneum and reach the viable epidermis where it functions, the formulation must be at a pH below 3.5. Above this pH, ionisation prevents penetration. This means the vehicle, pH, and packaging all directly determine whether a vitamin C product works.

A serum with 20% LAA in a poorly stabilised, high-pH base does less than a well-formulated 10% serum in an airless pump at pH 3.0–3.5.

Derivatives — the trade-offs

Vitamin C derivatives (ascorbyl glucoside, sodium ascorbyl phosphate, ascorbyl tetraisopalmitate) are more stable than LAA. They are less irritating and easier to formulate with. The trade-off is conversion: they must be enzymatically converted to ascorbic acid in the skin. The conversion efficiency varies by derivative, individual, and formulation. The clinical evidence for derivatives is generally weaker than for LAA, though improving for some (particularly ascorbyl glucoside).

The question of LAA versus derivatives is not "which is better" in the abstract — it is which delivers an effective dose of ascorbic acid to the viable epidermis given the formulation and concentration used.

How to use it

Vitamin C is most effective applied in the morning. It works synergistically with SPF — LAA depleted by UV exposure takes 72 hours to regenerate in skin cells, so daily morning application maintains a continuous antioxidant reserve.

Apply to cleansed, dry skin before moisturiser. LAA serums require a brief dry-down period; they should not be applied to wet skin, which dilutes the active and raises the effective pH.

Vitamin C and niacinamide can be layered. The older claim that they form a yellow complex (niacin) and reduce each other's efficacy is not supported by current evidence at the concentrations used in commercial skincare. They are compatible.

The Lux & Glo position

The niacinamide serum addresses the inflammatory and barrier-strengthening functions that are the appropriate starting point for most people. Vitamin C — applied as a well-formulated LAA serum in the morning — is a complementary active rather than a foundational one. It is most useful for people who already have a functioning barrier and stable routine and want to add a targeted intervention for tone, hyperpigmentation, or photoprotection.

The principle remains: one variable at a time, introduced after the baseline is established.