Is L-Cysteine safe in cosmetics?
L-Cysteine has a safety rating of "GOOD" in our database. EU status: permitted. US status: permitted.
GOOD
Active Ingredient
L-Cysteine
Also known as: (2R)-2-Amino-3-sulfanylpropanoic acid, NSC 8746, (R)-2-Amino-3-sulfanylpropanoic acid, Cystein, alpha-Amino-beta-mercaptopropanoic acid, L- (+14 more)
INCI: CYSTEINE
L-Cysteine (CAS 52-90-4) is a cosmetic active ingredient functioning as amino acid; reducing agent; hair-restructuring active; keratin smoothing agent;. NOAEL 2.33333 mg/kg bw/day (cross-referenced against ToxValDB ECHA IUCLID); EU Regulation 1223/2009 status: permitted. Industrial safety data is also available in the chemical safety database. Same-CAS public records also appear in industrial chemical safety, pharmaceutical, and food safety data.
Function
amino acid; reducing agent; hair-restructuring active; keratin smoothing agent; disulfide bond modulator
Editorial assessment
GOOD
Preliminary editorial note — not a computed safety verdict.
Chemistry & Identifiers
IUPAC Name
(2R)-2-amino-3-sulfanylpropanoic acid
InChIKey
XUJNEKJLAYXESH-REOHCLBHSA-N
Regulatory Status
EU EU Status
permitted
US US Status
permitted
US Notes
Permitted. Used in cysteine-based amino acid smoothing systems as a gentler, formaldehyde-free hair smoothing alternative. Modulates disulfide bonds via amino acid chemistry. No FDA restrictions.
For full compliance data across multiple jurisdictions, use the Substance Compliance tool.
Safety Data
Margin of Safety, dermal absorption, and sensitization profile summaries for L-Cysteine.
Calculate MoS for your specific formulation with the MoS Calculator.
Pre-Calculated Safety Assessment
Pre-calculated NOAEL → SED → MoS audit trail for L-Cysteine across SCCS product categories, with measured dermal absorption where available.
EPA ToxValDB — Toxicity Values
4 toxicity values from EPA ToxValDB (aggregated from CCTE, HPV, ToxRefDB, IRIS, and other regulatory dossiers).
| Endpoint | Value | Species | Route | Duration | Source |
|---|---|---|---|---|---|
| LOEL | =7 mg/kg-day | Rat | injection | short-term | ECOTOX |
| LD50 | >2000 mg/kg | Rat | dermal | acute | ECHA |
| LD50 | =660 mg/kg | Mouse | oral | - | ChemIDplus |
| LD50 | =1890 mg/kg | Rat | oral | - | ChemIDplus |
ECHA REACH Registration
European Chemicals Agency REACH dossier and Substances of Very High Concern listing.
Registration Type
Full
Tonnage Band
10 - 100 tonnes
Substance Type
SUBSTANCE
SVHC Candidate
No
PubChem Annotations
Safety and regulatory annotations from PubChem (NCBI) aggregated sources.
Safety (2)
Carcinogen Classification
No indication of carcinogenicity to humans (not listed by IARC).
Source: Toxin and Toxin Target Database (T3DB)
Skin Eye Respiratory Irritation
... A single ocular application of the test item L-Cysteine to rabbits at a dose of 0.1 g produced slight irritant effects, which were fully reversible within 48 hours. ... | ... The single dermal application of the test item L-Cysteine to three rabbits at a dose of 0.5 g showed neither irritant nor corrosive effects. ... | ... Single dermal application of the test item L-Cysteine to rats at a…
Source: Hazardous Substances Data Bank (HSDB)
Regulatory (2)
JECFA Number
1419
Source: WHO/JECFA
JECFA Evaluation
L-CYSTEINE | 2004 | No safety concern at current levels of intake when used as a flavouring agent | Not evaluated using the Procedure for the Safety Evaluation of Flavouring Agents; the substance is a macronutrient and a normal component of protein and, as such, human exposure through food is orders of magnitude higher than the anticipated level of exposure from use as a flavouring agent | TRS…
Source: WHO/JECFA
Function Categories & List Membership
Cosmetic/Chemical Function Categories
Filler
Flavouring and nutrient
Food preservative
Processing aids not otherwise specified
Regulatory List Membership (2)
EU Restricted for Children (Cosmetics)
FDA Substances Added to Food (EAFUS)
Editorial summary
Preliminary editorial summary for L-Cysteine — not a computed safety verdict. See the sourced toxicological data above.
Regulatory Flags
formaldehyde-free
amino-acid-smoothing
endogenous
gentle
disulfide-modulator
clean-label
professional-treatment
eu_children_restricted
fda_food_additive
jecfa_safe
Known Synonyms
(2R)-2-Amino-3-sulfanylpropanoic acid
NSC 8746
(R)-2-Amino-3-sulfanylpropanoic acid
Cystein
alpha-Amino-beta-mercaptopropanoic acid, L-
2-Amino-3-mercaptopropanoic acid, (R)-
alpha-Amino-beta-mercaptopropionic acid, L-
(R)-2-Amino-3-mercaptopropionic acid
alpha-Amino-beta-thiolpropionic acid, L-
EINECS 200-158-2
FEMA No. 3263
Half cystine
beta-Mercaptoalanine, L-
NSC-8746
UNII-K848JZ4886
(+)-2-Amino-3-mercaptopropionic acid
(2R)-2-amino-3-mercapto-propanoic acid
(2R)-2-amino-3-mercaptopropanoate
(2R)-2-amino-3-mercaptopropanoic acid
Chemical Function
Flavouring and nutrient
Processing aids not otherwise specified
Filler
Food preservative
Regulatory Lists
active_ingredient; Australia
inert_ingredient; non_food_use; Pesticides
pharmaceutical
children; Europe; Personal care
children; Europe; Personal care
pharmaceutical
Europe
food_additive; Substances Added to Food (formerly EAFUS) (2/2019)
Cross-Vertical Safety Context
Same-CAS public records found in pharmaceutical and food safety data.
SOURCE
ChEMBL / DrugCentral / FAERS
DB_PHARMA CAS match
Pharmaceutical Data
Same-CAS rows from the pharmaceutical spoke database.
Compound
CYSTEINE
ChEMBL ID
CHEMBL863
Max Phase
4
Adverse Event Associations
18
SOURCE
EFSA / FDA GRAS
food_additives
Food Safety Data
Same-CAS food additive or FDA GRAS records from the core public database.
Food Name
Cysteine
ADI
No safety concern at current levels of intake when used as a flavouring agent mg/kg bw
Food Category
preservative
Frequently Asked Questions
Is L-Cysteine allowed in the EU?
L-Cysteine EU regulatory status: permitted. This is based on EU Regulation 1223/2009 and its amendments.
What does L-Cysteine do in cosmetics?
L-Cysteine functions as: amino acid; reducing agent; hair-restructuring active; keratin smoothing agent; disulfide bond modulator. It is classified as a Active Ingredient in our database. CAS number: 52-90-4.
Is L-Cysteine used outside cosmetics?
L-Cysteine also appears in industrial chemical safety, pharmaceutical, and food safety data. The cross-vertical cards on this page render same-CAS public rows from the matched databases.
Compliance Tools
Check L-Cysteine compliance across multiple jurisdictions:
Ingredient Decoder
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82 Allergen Checker
Check for EU declarable fragrance allergens
Substance Compliance
Check compliance across multiple jurisdictions
MoS Calculator
Calculate Margin of Safety using SCCS methodology
Heavy Metal Limits
Compare limits across jurisdictions
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2-BROMO-2-NITROPROPANE-1,3-DIOL
MODERATE
Chemical family
Preservative
2-Bromo-2-Nitropropane-1,3-Diol (Bronopol)
MODERATE
Chemical family
Preservative (formaldehyde releaser)
Bronopol
MODERATE
Chemical family
Preservative
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