Cysteine
Sulfur-containing amino acid, antioxidant cofactor. Functional formulation ingredient with no standalone activity on the skin.
Evidence by route
Topical application
Inert formulation ingredient. Safe, no standalone activity.
How to read tiers A–F
- CWeak evidence. In vitro data, open-label studies, or expert consensus
Tolerability and safety
- Irritation potential
- Low
- Allergen risk
- Low
- Pregnancy
- Safe
About this ingredient
Cysteine is a sulfur-containing amino acid, non-essential in the human body (synthesized from methionine). One of the 20 standard amino acid protein building blocks. The thiol (-SH) group makes cysteine a key antioxidant cofactor. Mechanism in cosmetics. In formulas it acts as a functional ingredient: supplements the amino acid profile, supports the stability of other thiol-containing molecules (e.g., glutathione), can serve as a buffer antioxidant. No standalone topical skin activity – at cosmetic concentrations cysteine is more a structural element of the formula than an active. Where applied. Serums and creams with an 'amino acid complex' marketing angle, products for damaged hair, scalp masks. In Spain – mass-market shampoos labeled 'keratin and amino acids', niche serums with a barrier-restoring focus. Evidence base. Topical clinical data on skin are scarce. On hair, cysteine from keratin hydrolysates partially restores damaged sulfur bonds – confirmed in standard hair-strength tests. No comparable skin data. Safety. CIR confirmed L-cysteine safe in cosmetic use. Hypoallergenic, non-comedogenic. Reactions theoretically possible in patients allergic to sulfur or sulfonamides – extremely rare in practice. Pregnancy and lactation – safe. Cysteine is needed by the body at any life stage; topical cosmetic concentrations don't create issues. Best suited to: damaged hair – yes. For skin it's a filler component; don't expect a standalone effect.
Frequently asked questions
Is Cysteine safe during pregnancy?
Cysteine is considered safe during pregnancy at typical cosmetic concentrations. Systemic absorption through the skin is minimal.
What is Cysteine and what does it do in cosmetics?
Sulfur-containing amino acid, antioxidant cofactor.
What's the INCI name for Cysteine?
The INCI name is Cysteine.
See Cysteine in a product composition right on the store page
The Evigrade extension adds an evidence panel to iHerb, Sephora, Druni and 12 more stores. This ingredient and every other one in the product show evidence-tier, allergen risk and pregnancy/lactation flags at a glance.
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