Encyclopedia · supplement
L-cysteine
An amino acid that makes up much of hair keratin, with no controlled hair trial on its own.
What it is
Hair keratin is rich in cystine, two linked cysteines. That is why cysteine and methionine are marketed as hair building blocks.
How it works
Disulfide bonds between cysteines give hair its strength. Whether extra oral cysteine reaches follicles usefully when your diet already provides protein has not been shown.
What the studies show
- A 12-week assessor-blinded trial in 83 people tested a tablet with fish collagen, taurine, cysteine, methionine, iron and selenium as an add-on to drug treatment. Global assessment was 1.67 versus 0.66 for drugs alone. The contribution of cysteine cannot be separated. Milani 2023
- We found no controlled trial of cysteine alone for hair.
Side effects and safety
Generally tolerated at supplement doses. NAC can cause nausea and, rarely, hypersensitivity.
Bottom line
Dietary protein supplies cysteine. Treat any benefit from a blend as unproven for the individual amino acid. Discuss with a doctor.
Studies 1
Milani M et al. 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. 2023
Tested: tablet with marine collagen, amino acids, iron and selenium added to drugs vs drugs alone
Result: Global assessment 1.67 vs 0.66; assessor-blinded; contribution of individual ingredients cannot be separated.
Read the study ↗Related
- Marine protein supplements (Viviscal, Nourkrin)Branded fish- and shellfish-extract pills with positive but small, manufacturer-linked trials.
- Oral keratin supplementsKeratin capsules with one small company-linked trial and no sign that swallowed keratin reaches your hair.
- Collagen peptidesA protein powder with modest skin evidence and very little for hair.
- Hair supplement blends (Nutrafol, Viviscal and similar)Branded multi-ingredient hair vitamins that beat placebo in small, mostly company-linked trials.