Niacinamide and vitamin C "cannot be mixed"

The claim

Niacinamide and vitamin C neutralise each other, or convert to nicotinic acid and cause flushing, so they must never be layered.

What the evidence actually shows

This myth has an unusually traceable origin, and reading the source papers dissolves it.

The earliest, in Proceedings of the Society for Experimental Biology and Medicine in 1944, reports that a canary-yellow colour forms almost immediately when ascorbic acid and nicotinamide are moistened and stirred. That is the entire observation - a colour reaction. No loss of biological activity is reported, and no generation of nicotinic acid at cosmetic concentrations.

A 1963 study in the Journal of Pharmaceutical Sciences characterised it properly: a 1:1 charge-transfer complex forms in aqueous solution, with association pH-dependent and maximal near pH 3.8 between protonated nicotinamide and ascorbate anion. Again, a dilute-aqueous-solution phenomenon, not a demonstration that either ingredient is inactivated on skin.

There is one genuine and much narrower finding. A 2018 kinetic study found nicotinamide acts as an electron acceptor accelerating ascorbic acid photodegradation in aqueous solution under UV. That is a real formulation-stability consideration - aqueous, light-exposed, prolonged contact - and it is an argument about how to formulate and package a product, not about whether you may apply two products in sequence.

What actually follows from this

For a formulator: do not combine high concentrations of L-ascorbic acid and niacinamide in a single light-exposed aqueous system without stability testing. Reasonable.

For a user: layering a vitamin C product and a niacinamide product is fine. They are separate formulations, applied minutes apart, on skin rather than in a beaker, and any complex that forms is a colour change rather than a loss of function. The two are widely combined in commercial products for exactly this reason.

What to do instead

Use both. If you are unusually flush-prone and want to be cautious, separate them by time of day - but understand you are managing a theoretical concern, not an established interaction.