Nordihydroguaiaretic Acid

Nordihydroguaiaretic Acid (NDGA) is a potent polyphenolic antioxidant/anti-inflammatory used at low concentrations in topical formulas, but it is an active botanical-derived molecule rather than an inert excipient. Human patch-testing and case reports with NDGA-containing products (e.g., creosote bush derivatives) show that it can provoke irritant and occasional allergic contact dermatitis in reactive or barrier-impaired patients. Given its biologic activity and documented dermatitis potential, I score it as a notable irritant requiring cautious introduction and patch testing, especially in eczema-prone skin. Safety Notes: Nordihydroguaiaretic acid (NDGA) is an uncommon antioxidant/anti-inflammatory used mainly in niche OTC leave-on serums/creams and scalp products; in commercial INCI lists it is most often present at low, supportive levels around 0.01–0.1% due to solubility, odor/color, and irritation/sensitization considerations. High-strength consumer-available formulations marketed for oil-control/blemish or intensive antioxidant support have been observed up to ~1–2% (typically leave-on, often in hydroalcoholic/solubilized systems), while rinse-off use is rarer and generally kept at the low end because contact time is short and higher levels can increase irritation risk.