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L-cysteine (and L-cystine)

The sulphur amino acid that makes keratin hold its shape — with modest, mostly manufacturer-run evidence behind it.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsHair, skin & nailsKeratin building block

If protein is the raw material for hair, cysteine is the rivet. Keratin is unusually rich in this sulphur-containing amino acid, and it is cysteine’s sulphur atoms that form the disulphide bridges giving hair and nails their strength and springiness. That biochemistry is the honest rationale behind L-cysteine supplements — usually sold as the more stable dimer L-cystine, and usually bundled with B vitamins in the long-running “Pantogar / Priorin” family of products.

Key takeaways

  • Keratin is ~10–14% cysteine; its disulphide bonds are what let hair stretch and recover.
  • A controlled, double-blind trial of a B-vitamin + L-cystine combination improved anagen (growth-phase) rates and hair quality versus placebo in women with diffuse shedding.1
  • Almost all the evidence is on branded combinations and is manufacturer-run, so cystine’s solo contribution is hard to isolate.
  • Reasonable to trial for 3–6 months if you are shedding; honest expectation is modest.

The keratin connection

Cysteine carries a thiol (–SH) group, and two of them oxidise into a disulphide bond — the molecular cross-link that turns loose keratin chains into a tough, elastic fibre. It is the same chemistry a hairdresser breaks and reforms in a perm. Supply is the point: if the diet is short on sulphur amino acids (cysteine and methionine), the follicle has less to build with, which is the argument for supplementing them directly rather than as generic protein.

What the combination trials show

The cleanest evidence is the Budde/Tronnier line of work: a controlled, randomised, double-blind study in women with diffuse effluvium and hair-structure damage compared B-vitamin + L-cystine preparations against placebo and found the active formula significantly improved the anagen rate (proportion of growing hairs) and a measure of hair quality.1 Similar findings underpin the marketing of Pantogar-type products. The honest caveats: the trials are on fixed combinations (so you cannot credit cystine alone), they are small, and they are tied to the manufacturers.

Cysteine, cystine or NAC?

L-cystine (two cysteines joined) is the usual supplement form — stable and what the hair trials used. L-cysteine is the free amino acid. N-acetylcysteine (NAC) is a well-absorbed cousin used mainly as a glutathione precursor and mucolytic, not a dedicated hair product; do not assume the hair data transfers to it.

Worth a try?

If you shed diffusely and eat reasonably, a B-vitamin + L-cystine combination is a low-risk thing to trial for three to six months — and, in the spirit of this site, stop if nothing has changed by then. It is a supporting player, not a cure, and it will do nothing for pattern (androgenetic) hair loss, which is a different problem.

Bottom line: cysteine is a real keratin building block with modest, manufacturer-led evidence in combination formulas — a fair 3–6 month trial for diffuse shedding, not a fix for pattern baldness.

References

Adult research summary for education, not a prescription. These are food supplements, not treatments for disease.

  1. Budde J, Tronnier H, Rahlfs VW, Frei-Kleiner S. Systemic therapy of diffuse effluvium and hair structure damage. Hautarzt 1993;44(6):380–4. pubmed.ncbi.nlm.nih.gov

This article summarises nutrition science for education. It is not a substitute for individual medical advice; consult a professional before starting a supplement, in pregnancy, or alongside medication.