Options at a glance
| Option | Best for |
|---|---|
| L-ascorbic acid, 10–15%, airless pump | Best evidence |
| Sodium ascorbyl phosphate, 5–10% | Sensitive skin |
| Ascorbyl glucoside, 2–5% | Gentle daily use |
| Vitamin C in a dropper bottle, high concentration | Nothing, in our view |
A serum is mostly water, a humectant and an active. That means the useful questions are narrow: which form of vitamin C, at what concentration, in what packaging, and at what cost per millilitre.
Everything else on the box is decoration.
Which form
L-ascorbic acid is the form most of the research uses. It is also the least stable — it oxidises in air and light, turning orange-brown as it degrades. Effective, but demanding.
Sodium ascorbyl phosphate is a salt that the skin converts. More stable, much less irritating, and the evidence is reasonable, though not as deep as for L-ascorbic acid.
Ascorbyl glucoside is gentler still. Useful if you react to everything, but you need to accept a milder effect.
“Vitamin C” with no form named is the one to avoid. If the label does not tell you which molecule you are getting, you cannot compare it to anything.
Concentration
For L-ascorbic acid, the useful range is roughly 10% to 20%. Below 8% and the effect is hard to justify; above 20% you mostly get more irritation, not more benefit. A “30% vitamin C” claim on the front of a box is not a flex.
For the salt forms, 5% to 10% is the common range.
Packaging matters more than the price
Vitamin C oxidises. An airless pump or an opaque tube slows that down. A clear glass dropper bottle does not — every time you open it, air and light get in, and by week six the serum has often turned amber.
This is why the cheapest well-packaged serum beats the most expensive badly-packaged one. You are buying stability as much as concentration.
Test it yourself: if the liquid has turned noticeably orange or brown, it has oxidised. That is not a formula defect, it is a packaging one.
Cost per millilitre
Serums are sold in 15 ml, 30 ml and 50 ml bottles at wildly different prices. Divide the price by the volume. A cheaper 30 ml bottle can cost less per millilitre than a pricier 15 ml one, and the labels are designed so you do not notice.
What we would buy
The L-ascorbic acid option in an airless pump is the best-supported choice if your skin tolerates it. If it stings, drop to sodium ascorbyl phosphate. If you are buying a clear dropper bottle at a premium price, you are paying for the brand, not the vitamin C.