L-Cysteine and L-Cystine for Hair: What the Research Shows

Cysteine is a sulfur amino acid central to hair keratin. L-cystine, its oxidized form, appears in hair supplements. The best evidence is for combination products: an 80-person placebo-controlled trial of a supplement containing L-cystine increased hair density, and a 12-week trial of a supplement with cysteine, methionine and taurine improved ratings alongside drug treatment. Cysteine on its own hasn't been tested for hair.

Here's what the studies found and what they can't tell you.

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Key Takeaways

• Reviews describe cysteine as a key amino acid central to hair growth. Dysregulated cysteine metabolism is linked to hair breakage in alopecia areata.
• In an 80-person, 6-month, placebo-controlled trial, a supplement of L-cystine, saw palmetto, pumpkin seed, pygeum, vitamins and micronutrients increased hair density in chronic telogen effluvium and androgenetic alopecia.
• In an 83-person trial, a supplement with cysteine, methionine, taurine, collagen, iron and selenium taken with drug treatment improved hair loss ratings more than drug treatment alone.
• Both trials tested combinations, so the effect of cysteine or cystine alone is unknown.

What Is L-Cysteine?

Cysteine is a sulfur-containing amino acid that reviews describe as central to hair growth. L-cystine is two cysteine molecules joined together.

Do Cysteine Or Cystine Supplements Help Hair?

Not shown on their own. The positive trials used multi-ingredient formulas, and the hair-breakage research is a review of mechanisms.

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Using Cysteine Sensibly

• Treat single-ingredient claims as unproven.
• See a dermatologist for the cause of hair loss.
• Ask your doctor before taking supplements, especially if you're pregnant, breastfeeding, have a condition or take medication.

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What The Research Shows

Cysteine And Hair Breakage

A review of alopecia areata describes hair breakage and exclamation mark hairs as hallmarks of disease activity and points to dysregulated cysteine homeostasis, a key amino acid central to hair growth, as playing a pivotal role. It examines links to autophagy, ferroptosis, immunity and psychiatric aspects. It is a mechanism review, not a trial of supplements (Xu et al., 2024).

A Placebo-Controlled Combination Trial

Eighty patients aged 18 to 60 with chronic telogen effluvium or androgenetic alopecia were randomized to one capsule a day of a supplement containing L-cystine, Serenoa repens, Cucurbita pepo, Pygeum africanum, vitamins and micronutrients, or placebo, for 6 months. Hair density rose significantly more than with placebo (9.9 hairs/cm² at 3 months, 12.3 at 6 months). It was well tolerated (Piquero-Casals et al., 2025).

Another Multi-Ingredient Trial

In a 12-week randomized, assessor-blinded trial of 83 people with androgenetic alopecia or chronic telogen effluvium, adding a daily tablet of hydrolyzed fish collagen, taurine, cysteine, methionine, iron and selenium to drug treatment gave higher global assessment scores than drug treatment alone (1.67 vs 0.66 at week 12) (Milani et al., 2023).

Methionine is the related sulfur amino acid; see L-methionine and hair.

Taurine is a related sulfur-containing compound people ask about; see taurine and hair.

For the blend trials that include it, see amino acids for hair growth.

See also: Pygeum Bark for Hair: What the Research Shows

Frequently Asked Questions

Does L-cysteine help hair growth?

In combination supplements, it was part of formulas that improved hair measures in trials. Cysteine alone hasn't been tested for hair.

What is the difference between cysteine and cystine?

Cystine is two cysteine molecules joined together.

Is cysteine safe?

Talk to your doctor before taking supplements.

References

1. Xu W, Xie B, Wei D, et al. Dissecting hair breakage in alopecia areata: the central role of dysregulated cysteine homeostasis. Amino Acids. 2024. PubMed
2. Piquero-Casals J, Saceda-Corralo D, Aladren S, et al. Oral Supplementation with l-Cystine, Serenoa repens, Cucurbita pepo, and Pygeum africanum in Chronic Telogen Effluvium and Androgenetic Alopecia: A Double-Blind, Placebo-Controlled, Randomized Clinical Study. Skin Appendage Disord. 2025. PubMed
3. Milani M, Colombo F, GFM‐O‐Trial Investigators Group. Efficacy and tolerability of an oral supplement containing amino acids, iron, selenium, and marine hydrolyzed collagen in subjects with hair loss (androgenetic alopecia, AGA or FAGA or telogen effluvium). A prospective, randomized, 3-month, controlled, assessor-blinded study. Skin Res Technol. 2023. PubMed