Niacin

Niacin (nicotinic acid, vitamin B3) is primarily used as a vitamin/skin-conditioning agent, but at typical cosmetic-use levels it can trigger vasodilatory flushing, stinging, and warmth—reactions that are more pronounced in rosacea-prone and barrier-impaired (eczema) skin. Patch testing and real-world reports show it is less predictable than niacinamide, with irritation more likely when combined with other actives or low-pH routines. Given sensitive-skin safety considerations and the potential for notable immediate reactivity, a moderate irritancy score is warranted. Safety Notes: In commercial skincare, niacin (nicotinic acid) is used at very low levels (~0.01–0.1%) in sensitive-skin moisturizers/serums to minimize flushing while supporting barrier and tone benefits, and it appears similarly low in many rinse-off cleansers. The upper end of OTC consumer products is typically capped around ~0.5–1.0% because higher levels significantly increase irritation/flushing risk and are uncommon outside professional or drug-type uses; most leave-on products marketed for efficacy cluster below this ceiling.