Where You Keep Your Skincare Changes How Long It Works

Most skincare is robust. A cleanser does not much mind where it lives. Two categories are genuinely fragile, though, and both are the expensive ones: vitamin C and vitamin A.

Vitamin C is difficult on purpose

Pure L-ascorbic acid only crosses into skin under narrow conditions. Pinnell and colleagues found it has to be formulated below pH 3.5 to be absorbed at all, and that the useful ceiling for absorption is around 20 per cent. They also found that once skin levels are topped up, they fall with a half-life of about four days.¹

What makes it hard to formulate makes it easy to spoil. Gallarate and colleagues looked specifically at how ascorbic acid holds up in emulsions intended for cosmetic use, which is the question behind every vitamin C serum on a shelf.²

Newer derivatives are more stable but not indestructible. Swindell and colleagues showed that tetrahexyldecyl ascorbate, a widely used oil-soluble form, degrades quickly when exposed to singlet oxygen unless it is stabilised.³

This is the honest reason a vitamin C serum darkens over time. It is oxidising. A serum that has gone deep orange is not dangerous, but it is not doing what it did when you bought it.

Retinoids and light

Vitamin A derivatives have their own sensitivity. Brisaert and Plaizier-Vercammen investigated the photostability of a tretinoin lotion and how additives change it,⁴ and Del Rosso compared the photostability of two tretinoin formulations directly.⁵ Both are prescription formulations rather than the cosmetic strengths on our shelves, but the chemistry that makes vitamin A useful is the same chemistry that makes it reactive.

It is also why these products come in opaque tubes and airless pumps rather than clear jars, and why "decanting into something prettier" is a genuinely bad idea.

What actually to do

• Keep the lid on and the box if it has one. Air and light are the two things doing the damage.
• Not the windowsill, and not next to the shower. A bathroom that steams up twice a day is a warm, humid, bright cupboard.
• Buy the size you will use. A larger bottle is only better value if you finish it.
• Trust the colour. A vitamin C serum that has turned deep orange or brown has oxidised, whatever the date on the box says.

A cool, dark drawer in a bedroom is better than almost anywhere in a bathroom. The fridge is not necessary, and for oil-based products it can cause separation.

Ask us

If you are not sure whether something has turned, bring it in and we will look at it with you.

References

Each PMID resolves on Europe PMC, so you can read the paper rather than take our word for it.

1. Pinnell SR. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery, 2001. PMID 11207686
2. Gallarate M. On the stability of ascorbic acid in emulsified systems for topical and cosmetic use. International Journal of Pharmaceutics, 1999. PMID 10518678
3. Swindell WR. Tetrahexyldecyl Ascorbate (THDC) Degrades Rapidly under Oxidative Stress but Can Be Stabilized by Acetyl Zingerone. International Journal of Molecular Sciences, 2021. PMID 34445461
4. Brisaert M. Investigation on the photostability of a tretinoin lotion and stabilization with additives. International Journal of Pharmaceutics, 2000. PMID 10794926
5. Del Rosso JQ. Tretinoin photostability: comparison of micronized tretinoin gel and tretinoin gel. Journal of Clinical and Aesthetic Dermatology, 2012. PMID 22328956