Thioctic Acid

Thioctic acid (alpha-lipoic acid) is an antioxidant typically used around ~0.1–1% in topical products, and it has a documented propensity to cause stinging, erythema, or irritant reactions in reactive skin—especially when formulated at lower pH or in alcohol-heavy vehicles. While many users tolerate it, eczema-prone and barrier-impaired patients show a higher risk of irritation and occasional sensitization signals in patch testing/usage reports, so I score it as moderate with patch testing strongly recommended. Safety Notes: Thioctic acid (alpha-lipoic acid) appears in consumer skincare primarily as a leave-on antioxidant/anti-aging active, with low-end usage around 0.01–0.1% in multi-antioxidant serums and moisturizers where it is a supporting ingredient. Dedicated ALA serums/creams commonly sit around ~0.5–1%, while the highest-strength OTC specialty products marketed for intensive antioxidant/texture benefits reach about 3–5% (above this, stability/odor/irritation and formulation challenges become limiting in consumer cosmetics). It is uncommon in rinse-off products, and the observed market maximum is mainly in leave-on serums/creams with solubilization and stabilization aids.