Cystine
AKA: L-cystine, L-Dicysteine, Beta, Beta'-Dithiodialanine, L-Cysteine disulfide, 1-Cystine
Synthetic
Skin Conditioner
Hair Care
Amino Acid
Medicated Active
Not Clean
Cystine
Effective at 3.0%
Formula
C6H12N2O4S2
Mol. Weight
240.3
CAS #
56-89-3
Form
Powder
Functions
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.
Anti-Inflammatory
Moisturizing
Brightening
Reduces flyaways
Softens and detangles hair
Provides scent
Traditional Use
The oxidized dimer form of cysteine, discovered in the early 19th century. Used in modern haircare since the 1970s-1980s to strengthen hair structure and improve resilience, mimicking natural protein bonds in keratin.
EWG
4
CIR
Safe as used
Pregnancy
Yes
CIR Safety Findings
Not Phototoxic
medical-approval
all-shades-safe
Concentration Guide
0.0001%
6.0%
0%8%
Reported use
CIR reported use: 0.0001-5 | 0.0001-6 | 0.001-3
Regulatory Agencies
Skin Types
●all-skin-types
Best For
General
Pregnancy & Breastfeeding Safe
Yes
The paper trail
🔬Peer-reviewed1 paper →
⚖️Regulatory1 listing →
🛡️Safety-assessed1 review →
Counts, not a verdict. A listing isn't a study. The trail, you decide.
Verified Jun 2026
Data: PubChem · CosIng · EWG · CIR