Niacin is nicotinic acid, the other form of vitamin B3. It is closely related to niacinamide but behaves quite differently on skin, and the two should not be treated as interchangeable.
The flush
Niacin's distinctive property is vasodilation. It triggers release of prostaglandin D2, which widens blood vessels near the skin surface, producing a transient warmth, redness or tingling. The same mechanism is behind the well-documented flushing associated with oral niacin.
Onset is usually within minutes and it subsides on its own. It is a pharmacological response rather than an allergic one.
Why it appears in formulations
Some formulations include niacin deliberately, on the reasoning that increased local blood flow may improve delivery of other ingredients. Others include it as part of a broader vitamin B3 complex.
The evidence for penetration enhancement in skin specifically is thinner than the evidence for the flush itself.
Niacin versus niacinamide
Niacinamide does not cause flushing. If a product produces a warming or reddening sensation and both appear in the ingredient list, it is the niacin.
The two are not equivalent in skincare terms. Most of the well-evidenced benefits attributed to vitamin B3 in topical use, barrier support, pigment transfer, sebum regulation, come from research on niacinamide.
Who should be cautious
Anyone with rosacea or persistent facial flushing may find niacin uncomfortable, since it acts on precisely the mechanism they are managing.
