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Cysteine
AKA “L-cysteine, Cystein, (R)-2-Amino-3-mercaptopropanoic acid, Thioserine, Half-cystine”
Antioxidant
Ingredient Facts
Cysteine
AKA: L-cysteine, (R)-2-Amino-3-mercaptopropanoic acid, Thioserine, Half-cystine
Antioxidant · Antioxidant · EWG: 1
Synthetic
Antioxidant
Skin Conditioner
Hair Care
Amino Acid
Medicated Active
Clean
L-Cysteine
Formula
C3H7NO2S
Mol. Weight
121.16
CAS #
52-90-4, 3374-22-9
Form
Powder
See origin →
Functions
Antioxidant
Fights free radicals — the unstable molecules from UV, pollution, and stress that break down collagen and cause premature aging.
Antistatic
Hair Conditioning
A catch-all term for ingredients that improve how skin looks and feels — softer, smoother, more supple. The workhorse behind most moisturizers.
Hair Waving Or Straightening
Reducing
Neutralizes Free Radicals
Traditional Use
An amino acid first isolated in 1810. Discovered to support keratin formation in hair and skin; used since the 1970s-1980s in anti-aging and hair strengthening formulations for its role in collagen and glutathione synthesis.
Skin Types
●all-skin-types
Best For
General
Pregnancy & Breastfeeding Safe
Yes
EWG
1
CIR
Safe as used
Pregnancy
Yes
CIR Safety Findings
Not Phototoxic
medical-approval
all-shades-safe
Regulatory Status
USA✔ permitted
EU⚠ restricted
JAPAN⚠ restricted
CANADA⚠ restricted
Acetyl Cysteine
European Commission
EU Cosmetic Ingredient Database (CosIng)
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Cysteine
European Commission
EU Cosmetic Ingredient Database (CosIng)
→
Cysteine HCl
European Commission
EU Cosmetic Ingredient Database (CosIng)
→
Dimethiconol Cysteine
European Commission
EU Cosmetic Ingredient Database (CosIng)
→
Zinc Cysteinate
European Commission
EU Cosmetic Ingredient Database (CosIng)
→
Acetyl Cysteine
Environmental Working Group (EWG)
EWG Skin Deep Cosmetics Database
→
Cysteine
Environmental Working Group (EWG)
EWG Skin Deep Cosmetics Database
→
Cysteine HCl
Environmental Working Group (EWG)
EWG Skin Deep Cosmetics Database
→
Dimethiconol Cysteine
Environmental Working Group (EWG)
EWG Skin Deep Cosmetics Database
→
[Pharmacological studies of N-(2-mercapto-2-methylpropionyl)-L-cysteine (SA 96). VI. Effects on vitamin B6, metals and skin collagen in rats].
Yamauchi H, Morikawa K, Kuwano M, Hikida M, Fujimura K et al.
Nihon yakurigaku zasshi. Folia pharmacologica Japonica, 1985
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Verified Jun 2026
Data: PubChem · CosIng · EWG · CIR