Terpinen-4-ol on your skin: a safety profile

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(People-specific data is limited; this page draws from human adult context.) Terpinen-4-ol is the principal active monoterpene alcohol of tea tree oil (Melaleuca alternifolia, typically 30-48% by mass per ISO 4730 standard) and a minor constituent of marjoram, eucalyptus, and nutmeg essential oils. Demonstrated antimicrobial, antifungal, and anti-inflammatory activity; the scientific rationale for tea tree oil's topical use against acne and tinea pedis. ALSO the proximate agent of tea tree oil's well-documented cat toxicity: cats lack hepatic glucuronyl transferase activity needed to metabolize and excrete monoterpene alcohols, leading to rapid systemic accumulation, hypothermia, ataxia, tremors, hepatotoxicity, and death from concentrated tea tree oil exposure even at topical doses meant for dogs. Allergic contact dermatitis prevalence in humans is ~1-2% from oxidized tea tree oil (terpinen-4-ol itself is not a strong sensitizer, but its oxidation products are). Endocrine concerns from rare prepubertal-male gynecomastia case reports linked to tea tree + lavender oil shampoos are largely unsubstantiated at the terpinen-4-ol-specific level (the original 2007 NEJM case attributed the effect to tea tree + lavender combined, not isolated terpinen-4-ol).