Cysteine is a sulfur-containing amino acid that serves as the rate-limiting precursor for glutathione — the body's master antioxidant. Its thiol (-SH) group provides direct antioxidant activity through disulfide bond formation. In skincare, cysteine supports the skin's natural antioxidant defense system, contributes to keratin structure (hair and nails), and assists in brightening through glutathione pathway support. Often paired with glycine and glutamate for glutathione synthesis.
Key Skin Benefits
+ Rate-limiting precursor to glutathione synthesis
+ Thiol group — direct antioxidant activity
+ Supports natural skin antioxidant defense
+ Contributes to keratin structure (hair/nails)
+ Synergistic with glycine and glutamate for glutathione
Pros
+ + Glutathione precursor
+ + Direct antioxidant via thiol group
+ + Hair and nail strengthening
+ + Natural amino acid
+ + Supports skin's endogenous antioxidant system
Cons
− − Unstable — oxidizes rapidly
− − Strong sulfur odor
− − Limited research as topical monotherapy
− − Formulation challenges with oxidation
Specifications
Property
Value
Usage Areas
Face, Eye, Body, Hair
Product Type
Leave-On, Rinse-Off
Source
Synthetic / Fermentation
Natural Level
Natural-derived
Safety Ratings
Metric
Rating
Skin Sensitivity
2/5
Comedogenic Rating
0/5
EWG Score
1/10
Regional Regulatory Limits
Region
Leave-on %
Rinse-off %
Status
EU (CosIng)
No limit
No limit
Allowed
US (CIR/FDA)
No limit
No limit
Allowed
Japan (MHLW)
No limit
No limit
Allowed
ASEAN
No limit
No limit
Allowed
China (NMPA)
No limit
No limit
Allowed
Typical Usage % by Product Type
Product Type
Typical %
Serum
0.1 - 0.5%
Hair Treatment
0.5 - 2%
References
Lu SC — Glutathione synthesis pathway. Mol Aspects Med (2009). PMID: 18760456
Zhu H et al. — Cysteine in antioxidant defense. Amino Acids (2011). PMID: 20524036
Forman HJ et al. — Glutathione physiology. Free Radic Biol Med (2009). PMID: 18404961