L-Cystine
EVIDENCE: WEAKAlso sold as: cystine, L-cysteine (related), NAC (related), cystine dimer
L-cystine consists of two cysteine molecules joined together and occurs in proteins such as keratin. It differs from N-acetylcysteine, or NAC, which has much more research. Plain cystine has little human evidence as an oral supplement.
Compiled by iSupplementReviews editorial team ·Updated 2026-10-06 · 3 sources
- Type
- Amino acid (sulfur, non-essential)
- Studied dose
- No established supplement dose; NAC (the studied relative) uses 600–1,800 mg/day
- Best form
- NAC if the goal is glutathione support (the studied form); cysteine over cystine for bioavailability
- Upper limit
- No established upper intake limit — high cystine loads historically used to induce stones in animal models; dose caution applies
- Our rating
- WEAK — Mostly small, animal, or traditional evidence
What is L-cystine?
L-cystine consists of two cysteine molecules joined by a sulfur bond. It is a storage and transport form of the amino acid and contributes to keratin structures in hair and nails. Human research on cystine supplements is limited.
The NAC confusion
NAC and L-cystine have different evidence. NAC is used medically for acetaminophen overdose and mucus thinning and has hundreds of trials. L-cystine is less soluble and much less studied. A product containing cystine cannot rely on NAC research to establish its benefits.
What does the research show?
| Claim | Evidence | Dose studied | Verdict |
|---|---|---|---|
| Glutathione support | Via cysteine — but NAC is the studied delivery form | NAC: 600–1,800 mg/day | Evidence concerns a different compound |
| Hair/nail strength (keratin) | Biochemical rationale only | — | Unproven |
| Detox/antioxidant | Research concerns NAC | — | Evidence concerns a different compound |
| Oral/dental benefit | No data | — | Unproven |
What dose was studied?
No established dose; there are no meaningful human trials of oral L-cystine for any indication. The molecule’s solubility limits (and its kidney-stone history) make high dosing a poor idea regardless.
Side effects
At small blend doses, none expected. At high doses: digestive upset, and the historical quirk that cystine’s poor urinary solubility means excessive intake is a stone risk (it’s the same compound cystinuria patients crystallize).
Drug interactions
None well-documented at supplement doses; high-dose sulfur amino acids can theoretically affect drug metabolism marginally, but that’s not a demonstrated clinical interaction at these amounts.
Who should not take it
People with cystinuria, a genetic condition in which cystine forms kidney stones, should avoid extra cystine. People already eating adequate protein have no established need for a cystine supplement.
What the authorities say
No regulatory endorsement. The DRI treats methionine-plus-cysteine together as a requirement amply covered by dietary protein.
What to know about L-cystine
Cystine’s role in proteins does not establish a benefit from adding it to a supplement. NAC is a different molecule with a separate research record. An unspecified cystine amount cannot inherit those findings.
Frequently asked questions about L-Cystine
Is L-cystine the same as NAC?
No. NAC is a different, more readily absorbed compound studied for mucus thinning, glutathione support and treatment of acetaminophen overdose. L-cystine is less soluble and has little human supplement research. NAC findings do not establish benefits from L-cystine.
What does cystine do in the body?
Cystine is two cysteine molecules joined together. Cells can split it into cysteine for making proteins and glutathione. Cystine bonds also help give hair and nails their structure.
Will L-cystine help hair, nails, or skin?
Human trials have not established these benefits. Cystine contributes to keratin, but that biological role does not show that taking extra cystine improves hair, nails or skin. Dietary protein already supplies the amino acids used to make keratin.
Why is L-cystine in a dental supplement?
Human research has not established a dental benefit. Its role in proteins may explain the tissue-support claim, but that does not demonstrate an effect from a small amount in a dental blend.
Is L-cystine safe?
Small blend amounts are well below the high doses used to induce kidney stones in animal research. However, cystine is poorly soluble in urine, and people with cystinuria need particular caution. A blend amount alone does not establish safety for everyone.