What is the niacinamide and vitamin C mixing debate?

The debate stems from older research suggesting that niacinamide and vitamin C (L-ascorbic acid) react to form nicotinic acid, causing flushing and reduced efficacy. However, this reaction requires sustained high heat and does not occur meaningfully at room temperature on skin. Multiple modern studies and clinical products successfully combine both ingredients. If you want to be cautious, apply vitamin C in the morning and niacinamide at night.