About Cysteine
L-Cysteine is a semi-essential amino acid and the rate-limiting substrate for glutathione synthesis. It is distinct from NAC (N-Acetyl-L-Cysteine), which is the acetylated, more stable form preferred in supplements and clinical studies. Free-form L-Cysteine is less commonly supplemented due to rapid oxidation and instability, but appears in some amino acid blends and hair/nail formulas. Most clinical evidence for cysteine benefits comes from NAC studies rather than free-form cysteine. Contains a thiol group critical for keratin cross-linking in hair and nails.
What Cysteine supports
- Precursor to glutathione, the body's master antioxidant
- Provides sulfur for keratin in hair and nails
How much Cysteine to take
Clinical studies typically use 600–1800 mg of Cysteine. Limited RCT data for free-form L-Cysteine specifically. Doses extrapolated from NAC studies (600–1800 mg/day) where cysteine is the active moiety.
- Effective range
- 600–1800 mg
Clinical evidence
Moderate clinical evidence. Most clinical evidence comes from NAC studies rather than free-form cysteine; limited standalone data
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