Last verified: 17 days ago
L-Cystine
Also known as: L-Cystine, cystine, dicysteine, 3,3'-dithiobis(2-aminopropanoic acid)
Evidence under review. — Not yet rated
Amino acid found in hair and skin. May support hair growth and skin tone, but mostly studied in blends.
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What it does
L-Cystine is a naturally occurring amino acid made of two cysteine molecules linked together. It is a key building block of keratin, the protein that makes up hair and nails. Clinical studies have...
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Evidence quality
Evidence base hasn't been formally rated yet. See research below.
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Clinical dose
500 mg daily (skin lightening); 200 mg daily (fertility supplement blend); No established dose for hair loss as standalone
What the Science Says
L-Cystine is a naturally occurring amino acid made of two cysteine molecules linked together. It is a key building block of keratin, the protein that makes up hair and nails. Clinical studies have tested it primarily in multi-ingredient supplements for hair loss and skin lightening — one 12-week trial found a combination containing L-cystine reduced hair shedding by 59%, and another found 500 mg daily paired with glutathione lightened skin and reduced dark spots in Asian women. A small study also suggested it may boost flu vaccine response in nutritionally deficient elderly people when combined with L-theanine.
What It Doesn't Do
Won't regrow hair on its own — every positive hair study used it in a blend with other ingredients. No evidence it works as a standalone skin lightener. Won't improve male fertility — a large, well-designed RCT found an antioxidant blend containing L-cystine did not improve pregnancy rates and may have slightly reduced them. Not a proven immune booster for healthy adults. Don't confuse it with cysteine or NAC — they are related but different compounds.
Evidence-Based Benefits
A supplement blend with L-cystine reduced hair shedding by 59% and improved hair density over 12 weeks.
Weak EvidenceEffective at: Not established for L-cystine alone; used in combination product (2 capsules/day)
Supporting studies (click to view on PubMed):
500 mg L-cystine combined with glutathione lightened skin and reduced dark spots in Asian women over 12 weeks.
Weak EvidenceEffective at: 500 mg/day (combined with 250 mg glutathione)
Supporting studies (click to view on PubMed):
L-cystine with L-theanine may improve flu vaccine antibody response in elderly people with low protein or hemoglobin levels.
Weak EvidenceEffective at: No established dose from provided studies
Supporting studies (click to view on PubMed):
Absorption & Bioavailability
Unknown from provided studies — no pharmacokinetic data reported in the provided papers. Oral administration was used across all clinical studies without absorption data.
Red Flags to Watch For
- Almost always studied in multi-ingredient blends, making it impossible to isolate L-cystine's individual contribution
- A large RCT (1,171 men) found an L-cystine-containing antioxidant supplement did not improve pregnancy rates and was associated with lower ongoing pregnancy rates at 4–6 months
- Hair loss studies were open-label with no placebo control, making results difficult to interpret
- Skin lightening evidence is limited to Asian women; results may not generalize to other populations
- Many products in the NIH DSLD database contain L-cystine, but regulatory oversight of supplement blends is limited
Products Containing L-Cystine
See how L-Cystine is used in these analyzed products:
Frequently Asked Questions
What does L-Cystine do?
Amino acid found in hair and skin. May support hair growth and skin tone, but mostly studied in blends.
What is the effective dose of L-Cystine?
500 mg daily (skin lightening); 200 mg daily (fertility supplement blend); No established dose for hair loss as standalone
Is L-Cystine safe?
Almost always studied in multi-ingredient blends, making it impossible to isolate L-cystine's individual contribution
What doesn't L-Cystine do?
Won't regrow hair on its own — every positive hair study used it in a blend with other ingredients.
Research Sources
- PubMed
- NIH DSLD
This information is for educational purposes only and is not medical advice. Always consult a healthcare professional before starting any supplement regimen. Last updated: 2026-05-25