Kojic Acid

Kojic acid is a tyrosinase-inhibiting brightening active typically used around ~1–2% (sometimes higher), and it is well-documented to cause stinging, erythema, and irritant dermatitis in a meaningful subset of users, especially when combined with other actives or in low-pH systems. Patch testing and clinical use show it can be both irritating and a potential sensitizer/contact allergen, making reactions more likely in eczema-prone or barrier-impaired patients. Given its frequent intolerance in sensitive populations and the need for cautious introduction, it fits a significant irritancy risk at typical use levels. Safety Notes: In consumer skincare, kojic acid is commonly found in leave-on brightening serums/creams at low levels around 0.1–1% (often in combination formulas to improve tolerance and stability), and in some rinse-off cleansers/soaps around similar or slightly higher levels. High-strength OTC hyperpigmentation products marketed to the general public (not prescription/pro-only) are observed up to about 4% in leave-on creams/serums, with stability (oxidation/discoloration) and irritation risk typically limiting higher real-world use. Regulatory approaches vary by region, so the upper end reflects observed market availability rather than a universally permitted maximum.