Niacinamide

Four to eight weeks of daily use for marks and uneven tone, and roughly two to four weeks for barrier comfort and less shine. If nothing has changed after three months, the problem is not consistency: the product probably contains too little, or your goal needs a different active.

It's a form of vitamin B3 with four well-documented effects: it regulates sebum production, reduces the look of pores, fades dark spots and strengthens the skin barrier (boosting ceramide synthesis). That's why it appears in so many K-beauty serums and toners.

The clinical studies were mostly done at 2–5%, and that's where the evidence is. 10–20% concentrations are marketing: there's no proof they add benefit, but they do add risk of irritation and flushing.

Yes. The supposed incompatibility is a myth based on outdated lab conditions. With modern formulas they can be used together — in fact many brands combine them in the same product to target spots and tone.