Niacinamide
In plain terms
A form of vitamin B3 used to calm redness, even out tone, and support the skin barrier. It is one of the better-tolerated actives, and the flushing it sometimes gets blamed for usually belongs to its lookalike, niacin.
What the evidence shows
Niacinamide is a form of vitamin B3, and it has earned its reputation as one of the gentlest active ingredients in skincare. Safety reviewers found it produced no stinging up to 10%, no irritation up to 5%, and it was neither a sensitizer nor a photosensitizer; dermatologists use it for acne, rosacea, and pigmentation, and a related body of work shows it helping skin repair UV-induced DNA damage. The flushing it sometimes gets blamed for is mostly a case of mistaken identity. Niacin, a different form of B3, reliably causes a warm, red flush; niacinamide does not, and an old expert report that lumped the two together is the source of much of the confusion. When niacinamide itself does cause a brief flush, the usual reasons are a concentration above roughly 10% or a trace of leftover nicotinic acid in a less-pure batch, both of which are about the product, not your skin, and both transient. True allergy to it is documented but rare. Worth keeping separate from all of this are the oral high-dose supplement questions, which do not bear on a serum you apply and wash off. For topical use, niacinamide sits firmly in the no-real-concern column.
On the label
Listed as niacinamide or nicotinamide. If a product makes you flush where another did not, the likely culprit is concentration above roughly 10% or a trace of nicotinic acid from a less-pure batch, not your skin.