Cystine is a sulfur-containing amino acid (a dimer of cysteine) used in skincare and haircare as a conditioning agent and to support keratin structure and antioxidant defense. It is found naturally in skin and hair proteins and is generally well tolerated in topical formulations.
| Reported effect | How often | Notes |
|---|---|---|
| Mild skin irritation or redness | Rare | Occasional transient irritation at application site, typically in sensitive individuals. |
| Allergic contact dermatitis | Very rare | Isolated sensitization reports; uncommon for amino acid ingredients. |
Frequencies reflect typical cosmetic use reported in the literature, not a guarantee for your skin.
Peer-reviewed papers on this ingredient, via PubMed.