Cysteine
SIS-tee-een
What Is Cysteine?
A sulfur-containing amino acid that is one of the key building blocks of hair keratin. It forms disulfide bonds that give hair its strength and structure.
Source: Synthesized or extracted from animal hair/feathers
Origin: Semi-synthetic
Also known as: L-Cysteine, Cysteine HCl
What Does Cysteine Do to Your Hair?
Helps repair and strengthen the disulfide bonds in hair keratin that are broken by chemical treatments and heat damage. Penetrates the hair shaft to reinforce structural integrity. Used in some straightening treatments as a formaldehyde-free alternative.
Best For
Damaged hair, chemically treated hair, heat-damaged hair, breaking hair
Not Recommended For
Very fine hair, healthy hair (unnecessary)
Concentration Matters
0.1-2% in repair products; higher in straightening treatments
Hair Type Compatibility
Method Compatibility
Safety & Regulation
Safe at cosmetic concentrations. Used in some Japanese straightening treatments as a gentler alternative to formaldehyde-based keratin treatments. The sulfur content can have a mild odor during processing.
FDA Status: Generally Recognized As Safe
FDA considers safe for cosmetic use. Also approved as food supplement.
EWG Score: 2/10
HairAide Verdict
The most structurally important amino acid for hair health. Products containing cysteine offer targeted disulfide bond repair. Look for it in bond-building and repair treatments for genuinely damaged hair.
Products That Contain Cysteine
Bond repair treatments, keratin treatments, strengthening conditioners, protein treatments
Alternatives to Cysteine
If you want to avoid Cysteine, look for products with these ingredients instead:
Methionine, Keratin, Cystine
Frequently Asked Questions
Yes, Cysteine has a safety rating of 8/10 and is generally considered safe for use in hair products. Safe at cosmetic concentrations. Used in some Japanese straightening treatments as a gentler alternative to formaldehyde-based keratin treatments. The sulfur content can have a mild odor during processing.
Cysteine is CG-method approved and is generally considered safe for curly hair.
Yes, Cysteine is approved for the Curly Girl method.
Some alternatives to Cysteine include: Methionine, Keratin, Cystine
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