Cystine
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An amino acid formed from two molecules of cysteine joined together. It occurs naturally in dietary protein and in the body's own protein structures, including hair and skin keratin.
Why it’s used
Taken by some as a building block related to cysteine, sometimes marketed for hair, skin, or nail support, or as part of general amino acid formulas.
What the evidence says
There is little dedicated human clinical research on cystine supplementation itself, as distinct from its more commonly studied precursor form, N-acetylcysteine. Claims about hair or skin benefits from cystine supplementation specifically have not been well established in controlled trials.
Where we looked
There was no paper to cite here, so this is the search instead — run it yourself and see what we saw.
- PubMed: 31 records · searched August 2026
"Cystine"[Title/Abstract] AND ("dietary supplements"[MeSH] OR supplementation[Title/Abstract] OR supplement[Title/Abstract] OR supplements[Title/Abstract] OR "oral administration"[Title/Abstract]) AND (randomized controlled trial[pt] OR clinical trial[pt] OR meta-analysis[pt] OR systematic review[pt]) AND humans[MeSH] - PubMed: 12449 records · searched August 2026
"Cystine"[Title/Abstract]
This says what this database holds under these terms. It is not a finding that no research exists.
Context
No standard studied dosing protocol for cystine as a standalone supplement has been established in the available literature.
Educational information, not medical or safety advice. Consult a professional for allergies or medical concerns.