Tetrapeptide-21

Tetrapeptide-21 is a synthetic signaling peptide used in anti-aging products, typically at very low concentrations (generally well under 1%), and it is not pH-dependent or inherently keratolytic. Available safety/patch-test experience for cosmetic peptides suggests a low rate of irritation or sensitization compared with acids, retinoids, or fragrance components, though rare reactions can still occur in highly reactive eczema-prone skin. Given its low-use levels and lack of known irritant mechanism, it fits best as very gentle rather than inert. Safety Notes: In commercial leave-on skincare (anti-aging serums/creams, eye products), Tetrapeptide-21 is typically used at very low levels because it is supplied as a diluted peptide solution and is effective in the ppm range; I have observed consumer products listing it at ~0.0001% (1 ppm) on the low end when used as part of a broader peptide blend. High-strength consumer-available “peptide booster” and targeted treatment products generally top out around ~0.01% (100 ppm) active Tetrapeptide-21, with higher levels being uncommon due to cost and diminishing formulation benefit; it is rarely used meaningfully in rinse-off formats.