Cysteine is a sulfur-containing amino acid used in skincare and haircare for its antioxidant properties and its role in supporting keratin and glutathione synthesis. It is also employed in hair straightening and perming formulations to reduce disulfide bonds.
| Reported effect | How often | Notes |
|---|---|---|
| Mild skin irritation or redness | Uncommon | Can occur particularly at higher concentrations or on sensitive skin. |
| Allergic contact dermatitis | Rare | Sensitization reported in some individuals with patch testing. |
| Unpleasant sulfurous odor sensitivity | Uncommon | Cosmetic concern rather than a true adverse health effect. |
| Scalp or hair shaft damage with repeated use | Rare | Associated mainly with hair-relaxing applications involving bond breakage. |
Frequencies reflect typical cosmetic use reported in the literature, not a guarantee for your skin.
Peer-reviewed papers on this ingredient, via PubMed.