Amino Acid
L-Cystine
Also known as: Cystine, L-Cystine
Oxidized dimer form of cysteine, important for keratin production, hair/nail/skin health, and antioxidant support via glutathione synthesis.
Last reviewed June 2026 · Evidence graded from peer-reviewed research, not editorial opinion. Educational only — not medical advice.
Primary uses
- Hair health
- Nail strength
- Skin support
- Keratin synthesis
- Glutathione precursor
How it works
- Reduced to cysteine in the body
- Provides sulfur for keratin production
- Supports glutathione synthesis
- Structural component of hair and nails
Dosage
- Typical range
- 500-1500 mg daily
- Timing
- With meals
- With food
- Best with food
- Duration
- Safe long-term
- Special populations
- Hair/nail support
Forms
- Capsule
- Powder
Safety
Common side effects
- Well tolerated
Contraindications
- Cystinuria
Products containing L-Cystine
Top-scoring supplements in our catalog that list L-Cystine on the label. Each product is graded on Formulate's ingredient-level rubric — dose accuracy, form, transparency, and third-party testing.
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Evidence notes
Established role in keratin synthesis and sulfur metabolism
Grade B: Some human trials support key claims; further confirmation needed.
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Medical disclaimer. This page is educational and does not replace advice from a qualified healthcare provider. Individual needs and risks vary — consult a professional before starting, stopping, or combining supplements, especially if you are pregnant or nursing, taking medication, or managing a health condition.