The persistent rumour that niacinamide cancels out vitamin C (or worse, generates niacin and turns your face red) is repeated in beauty forums every week. It comes from one specific situation in the 1960s and stopped being relevant about thirty years ago.
Where the myth started
The original observation was in pure-form, unstabilised niacinamide combined with unbuffered L-ascorbic acid at high concentrations, heated, in a lab setting. Under those conditions, a small amount of niacin (nicotinic acid) was produced, which can cause transient flushing in some people.
None of those conditions describes a finished skincare formulation. Reputable products buffer the pH, stabilise both molecules, and use percentages well below the threshold where the reaction matters even theoretically.
What modern formulation does differently
L-ascorbic acid is now routinely combined with antioxidants and pH-stabilisers (ferulic acid, vitamin E) that preserve its activity for months instead of weeks. Niacinamide is sold at 4-10% in stable, mildly acidic vehicles that don't trigger conversion.
The skin's own pH (~5.5) and the typical formula pH (3.5-5.5) sit well outside the range where the 1960s reaction occurs. Even if a small amount of niacin formed in your bottle, it would be too dilute to cause flushing once spread over the face.
Studies that put the question to rest
A 2017 in vitro study published in the Journal of Cosmetic Science applied 10% niacinamide and 10% L-ascorbic acid simultaneously, then tested for niacin conversion at 24 and 72 hours. No detectable conversion at typical formula pH. A separate clinical study on combined-use efficacy found no reduction in vitamin C antioxidant activity when applied with niacinamide on the same routine step.
How to actually combine them
Both at once
Apply L-ascorbic acid first (lowest pH), wait 30-60 seconds, then niacinamide. The wait isn't because of any interaction — it's just to let each layer absorb. The Inkey List C-50 Booster plus Naturium Niacinamide 12% Serum is the cheap UK pairing that works.
Morning vs evening split
Some people prefer vitamin C in the morning (for daytime antioxidant protection against UV-induced free radicals) and niacinamide in the evening. This works fine but isn't necessary for chemistry reasons.
If you do see flushing
Patch test the individual products. Flushing on combined use is almost always reaction to one product alone (often the C, which can sting compromised barriers), not an interaction.
The combination is well-documented as safe and complementary. Vitamin C handles oxidative stress; niacinamide reduces inflammation, regulates sebum, and supports the barrier. Together they cover more ground than either alone.