Quick answer
Vitamin C is the most oxidation-prone ingredient on your shelf. Choosing well is mostly about form and packaging, not price.
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1. Pick the form before the percentage
L-ascorbic acid — the strongest evidence, and the least stable. Usually 10–20%. Can sting.
Sodium ascorbyl phosphate / magnesium ascorbyl phosphate — gentler, more stable, slower results.
Ascorbyl glucoside / THD ascorbate — stable derivatives, better for sensitive skin.
Beginners and sensitive skin do better starting with a derivative.
2. Strength: 10% is plenty to start
10% is plenty to start. Above 20% adds irritation, not results.
3. Packaging matters more than the percentage
Vitamin C oxidises in light and air. Buy opaque, airtight, small. A clear dropper bottle of L-ascorbic acid is losing potency on the shelf.
4. How to tell it has oxidised
Deep yellow to brown means oxidised. It is not dangerous, but it no longer works, and heavily oxidised vitamin C can stain fabric.
5. Storage in Bangladesh's heat
Store it away from the bathroom. Heat accelerates oxidation, and a Dhaka bathroom in summer is the worst place for it.
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