Vitamin C is one of the better-researched skincare antioxidants, and a deservedly popular one. But it is also notoriously temperamental, and many people buy a serum, use it for months, and never realise it stopped working weeks ago. Understanding what it does — and why it degrades — is the difference between benefit and wasted money.
The ingredient is genuinely good. Keeping it effective is the real challenge.
What it can do
As an antioxidant, vitamin C helps neutralise some of the daily damage from environmental stressors like pollution and UV, complementing rather than replacing sunscreen. Over consistent use it is associated with a brighter, more even tone and support for the skin’s firmness, since it plays a role in the processes behind collagen.
It is best thought of as a daytime protective and brightening step, not an overnight transformation. Its rewards are cumulative and subtle.
Why it stops working
Vitamin C, especially in its most-studied form, is unstable: exposure to air, light and heat gradually oxidises it, turning it less effective — and often turning the serum yellow, orange or brown. A serum that has darkened noticeably has likely lost much of its potency, no matter how it smells or feels.
This is why packaging matters. Opaque, air-restricting bottles protect the ingredient; clear droppers left open on a sunny shelf destroy it.
Getting the most from it
Store it cool, dark and closed, and do not buy more than you will use before it degrades. Introduce it gradually, as some formulas can tingle at first, and pair it with daily sunscreen, since the two work well together against environmental damage.
If your serum has turned dark, replace it rather than hoping. And remember its ceiling: vitamin C is a helpful supporting act, not the star that replaces the fundamentals of sun protection and a steady routine.