Spermidine

Spermidine is a naturally occurring polyamine used in skincare mainly as an anti-aging/cell-support ingredient, typically at low concentrations (often well under 1%). While not a classic irritant like acids or retinoids, polyamines are biologically active and can sting or provoke reactivity in compromised barriers (e.g., eczema, post-procedure) even when overall tolerability is good in the general population. Given limited large-scale human irritation datasets in highly sensitive cohorts and the need to err on patient safety, I rate it as mild with occasional sensitivity possible. Safety Notes: In consumer skincare, spermidine is typically used as a low-dose bioactive/anti-aging claim ingredient, most often in leave-on serums and creams at microdose levels (commonly around 0.001–0.05%), with some products listing it near the end of the INCI at ~0.0001% as part of multi-active complexes. High-strength niche formulations marketed for “cell renewal/autophagy” can reach the low tenths of a percent, with the upper end around ~0.5% limited by odor, stability (oxidation/amine reactivity), and irritation/tack risks; rinse-off uses are rare and generally lower due to short contact time. There is no specific FDA/EU maximum limit established for spermidine in cosmetics, so observed market practice and formulation constraints primarily set the range.