Why it changes colour
L-ascorbic acid, the most studied form of vitamin C, is unstable. Air, light and heat oxidise it, and as it breaks down, the serum shifts from clear or pale yellow to dark yellow, then orange or brown.
Clearto pale yellow: fine
Dark yellowlosing strength
Orangeto brown: oxidised. Replace it.
Is oxidised vitamin C harmful?
It’s mainly less effective. Some people find oxidised serum can leave a temporary yellowish tint on the skin. Either way, you’re no longer getting what you paid for.
Air, light and heat are the enemy.
Make it last
- Choose opaque or dark, airtight packaging.
- Close the cap straight away; store away from sunlight and heat.
- Buy smaller bottles you’ll finish within a few months.
- Derivatives (like sodium ascorbyl phosphate or ascorbyl glucoside) are more stable, though generally gentler.
General guidance. Follow the product’s own storage and expiry information.