নিয়াসিনামাইড না ভিটামিন সি

Both get described as "brightening", which flattens two quite different ingredients into one shelf.

Niacinamide

A form of vitamin B3. Stable, well tolerated, and useful across several concerns at once — oil balance, the look of pores, barrier support and tone.

• Typical strengths: 2% to 10%
• Time of day: morning or night
• Tolerance: high; 10% can be too much for sensitive skin
• Pairs comfortably with almost everything

Vitamin C

Usually L-ascorbic acid. More powerful as an antioxidant, and less stable — it degrades in light and air, which is why good ones come in opaque or airless packaging.

• Typical strengths: 10% to 20%
• Time of day: morning, under sunscreen
• Tolerance: moderate; can sting on compromised skin
• Store away from sunlight, and replace it if it turns dark orange

Can you use both?

Yes. The old advice that they cancel each other out came from lab conditions that do not reflect how formulas are actually made. If you want to keep it simple, use vitamin C in the morning and niacinamide at night.

Which first?

If your skin is sensitive or you are new to actives, start with niacinamide. If dullness is your main concern and your barrier is in good shape, vitamin C is the stronger tool.

Neither will do much without daily sunscreen.