Evidence-based·Last reviewed May 31, 2026·How we grade evidence

L-Cystine

Amino-acidBest with a meal

A stable dimer of the amino acid L-cysteine, used as a glutathione precursor in the body. Most modern human evidence is for the L-cystine + L-theanine combination (often Japanese-origin trials) supporting immune function around vaccination, intense exercise, and surgery. L-cystine alone has limited modern clinical evidence — N-acetylcysteine is the more widely studied cysteine donor.

Quick decision guide

May help most

Adults wanting a glutathione-precursor option, especially for short-term immune support around vaccination or heavy training — typically as part of an L-cystine + L-theanine combo.

Common dosing range

250–700 mg L-cystine per day. The L-cystine + L-theanine RCTs used 700 mg cystine + 280 mg theanine daily.

When to expect effects

10–14 days for vaccine response and cold-incidence outcomes; weeks for hair/nail effects (if any).

Watch out for

Avoid in cystinuria (a rare genetic disorder of cystine kidney-stone formation). N-acetylcysteine has better safety data and broader clinical use as a cysteine donor — consider it as an alternative.

Evidence snapshot

Influenza vaccine response (with theanine)Emerging

Cold prevention in stressed athletes (with theanine)Emerging

Hair, skin, nailsLow

L-cystine alone (no theanine)Low

What is it

L-cystine is the oxidized dimer of the amino acid L-cysteine, joined by a disulfide bond. It is used as a stable supplement form that the body reduces to cysteine.

Is it worth it for you?

Use this as a quick fit check, not a diagnosis.

Worth considering if…

You're an older adult or immunocompromised and want to try a low-risk add-on around your annual flu vaccination, alongside the vaccine itself

You're an athlete in a heavy training block and prone to colds during intensification — an L-cystine + theanine combo for 10 days has small-trial support

You're considering hair/nail combination products and want to know how cystine fits in (it's part of keratin, but standalone evidence is thin)

You don't tolerate NAC's sulfur taste/odour and want a cysteine-donor alternative

Probably skip if…

You have cystinuria — supplemental cystine worsens stone-forming risk

You expect dramatic 'detox', anti-aging, or athletic-performance benefits — evidence doesn't support those framings

You're already taking N-acetylcysteine — NAC delivers cysteine more efficiently and has more clinical evidence

You're pregnant or breastfeeding — supplement use beyond dietary intake isn't well studied

You're hoping a hair/nail supplement will reverse genetic or hormonal hair loss — it won't

Evidence at a glance

Goal	Effect	Best fit	Time
Influenza vaccine response in older adults Limited Evidence	Higher seroconversion and seroprotection rates vs placebo at standard flu-vaccine doses	Older adults receiving influenza vaccination, especially those at high risk of suboptimal response	Started 4 days before vaccination, continued for 10 days after
Common cold prevention in stressed athletes Limited Evidence	Reduced cold incidence and preserved NK-cell activity vs placebo during intensified training	Endurance athletes during heavy training blocks	Within a 10-day supplementation window during the training stress period
Post-surgical inflammation and infection Limited Evidence	Reduced CRP, WBC, and infection rate vs control in a single small open-label trial	Elective surgery patients in research settings	Days (4-day pre-op course)
Hair, skin, and nails Mixed Evidence	Not quantified for L-cystine alone	People with marginal protein intake or chronic GI malabsorption — and only as part of overall protein adequacy	Months for hair (hair-shaft growth rate is the rate limit)
Glutathione status and oxidative stress Mixed Evidence	Not quantified for L-cystine alone	Not established in modern human trials	Not established for L-cystine

Influenza vaccine response in older adults

Limited Evidence

Effect: Higher seroconversion and seroprotection rates vs placebo at standard flu-vaccine doses
Best fit: Older adults receiving influenza vaccination, especially those at high risk of suboptimal response
Time: Started 4 days before vaccination, continued for 10 days after

Common cold prevention in stressed athletes

Limited Evidence

Effect: Reduced cold incidence and preserved NK-cell activity vs placebo during intensified training
Best fit: Endurance athletes during heavy training blocks
Time: Within a 10-day supplementation window during the training stress period

Post-surgical inflammation and infection

Limited Evidence

Effect: Reduced CRP, WBC, and infection rate vs control in a single small open-label trial
Best fit: Elective surgery patients in research settings
Time: Days (4-day pre-op course)

Hair, skin, and nails

Mixed Evidence

Effect: Not quantified for L-cystine alone
Best fit: People with marginal protein intake or chronic GI malabsorption — and only as part of overall protein adequacy
Time: Months for hair (hair-shaft growth rate is the rate limit)

Glutathione status and oxidative stress

Mixed Evidence

Effect: Not quantified for L-cystine alone
Best fit: Not established in modern human trials
Time: Not established for L-cystine

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Influenza vaccine response in older adults

Supplement benefit

Limited Evidence

Miyagawa et al. 2013 randomised 31 elderly nursing-home residents to L-cystine 700 mg + L-theanine 280 mg daily or placebo for 14 days starting 4 days before influenza vaccination. The active group showed significantly higher seroconversion and seroprotection rates to influenza A H1N1 and H3N2 strains. Mechanism proposed: cystine supplementation restores T-cell glutathione, which declines with age and constrains vaccine-driven antibody response.

Effect size

Higher seroconversion and seroprotection rates vs placebo at standard flu-vaccine doses

Time to effect

Started 4 days before vaccination, continued for 10 days after

Best fit

Older adults receiving influenza vaccination, especially those at high risk of suboptimal response

Less likely

Healthy young adults who consistently seroconvert from vaccination

Bottom line: Low-risk add-on with one small positive trial for older adults around flu shots — not a substitute for the vaccine itself.

Evidence is mixed

Small single-site Japanese trial (n=31); not replicated in large independent samples or in other vaccines.

SourceMiyagawa et al., 2013 — Geriatrics & Gerontology International· conflicting

SourceMiyagawa et al., 2013 — Geriatrics & Gerontology International

Common cold prevention in stressed athletes

Supplement benefit

Limited Evidence

Kurihara et al. 2010 randomised 35 college-age long-distance runners to L-cystine 700 mg + L-theanine 280 mg daily or placebo for 10 days during heavy training. The active group reported significantly lower cold incidence and showed less exercise-induced suppression of natural killer (NK) cell activity. Sample is small, single-centre, and the combination (not L-cystine alone) drove the effect.

Effect size

Reduced cold incidence and preserved NK-cell activity vs placebo during intensified training

Time to effect

Within a 10-day supplementation window during the training stress period

Best fit

Endurance athletes during heavy training blocks

Less likely

Sedentary adults; people not regularly exercise-immunosuppressed

Bottom line: Reasonable trial during a heavy training block; standard hygiene and rest still matter more.

SourceKurihara et al., 2010 — Journal of Amino Acids

Post-surgical inflammation and infection

Disease adjunct

Limited Evidence

Murakami et al. 2009 open-label RCT in 29 elective colorectal-surgery patients found pre-operative L-cystine 700 mg + L-theanine 280 mg for 4 days reduced post-surgical CRP, white-cell count, and infection rates vs control. Small open-label trial; treat as preliminary rather than practice-changing.

Effect size

Reduced CRP, WBC, and infection rate vs control in a single small open-label trial

Time to effect

Days (4-day pre-op course)

Best fit

Elective surgery patients in research settings

Less likely

Routine surgical care outside a research protocol

Bottom line: Promising small open-label signal — not part of standard pre-op nutrition protocols.

SourceMurakami et al., 2009 — Surgery Today

Hair, skin, and nails

Supplement benefit

Mixed Evidence

L-cystine is the dimer form of cysteine, a sulfur amino acid that makes up about 10–14% of keratin (the structural protein of hair and nails). Combination products bundle L-cystine with biotin, B vitamins, and millet seed extract, claiming improved hair density. Trials of these combinations exist but rarely isolate L-cystine's contribution. Standalone cystine supplementation has not been shown to reverse hair loss or improve nail strength in well-controlled studies.

Effect size

Not quantified for L-cystine alone

Time to effect

Months for hair (hair-shaft growth rate is the rate limit)

Best fit

People with marginal protein intake or chronic GI malabsorption — and only as part of overall protein adequacy

Less likely

Anyone with androgenetic alopecia, alopecia areata, or hormonally-driven hair loss

Bottom line: Plausible as part of broader protein adequacy; standalone L-cystine isn't a hair drug.

SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

Glutathione status and oxidative stress

Mechanism only

Mixed Evidence

L-cystine is reduced to L-cysteine in the body and feeds the gamma-glutamyl-cysteine ligase step of glutathione synthesis — the rate-limiting reaction. Mechanistically a glutathione precursor, but most clinical evidence for glutathione manipulation uses N-acetylcysteine (NAC), oral glutathione itself, or whey protein (rich in cysteine). Standalone L-cystine trials measuring glutathione outcomes in humans are scarce.

Effect size

Not quantified for L-cystine alone

Time to effect

Not established for L-cystine

Best fit

Not established in modern human trials

Less likely

Anyone seeking a well-validated glutathione precursor — NAC has more clinical evidence

Bottom line: Plausible glutathione precursor; if that's the goal, NAC has the better human evidence base.

SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

How it works

L-cystine is reduced in the gut and tissues to two cysteine molecules. Cysteine is the rate-limiting amino acid for glutathione synthesis and also contributes to the disulfide bonds that stabilize keratin in hair and nails. As a supplement, cystine raises cysteine and glutathione availability similar to N-acetylcysteine (NAC), though NAC has a more established clinical use.

How to take it

1. Typical dose

• 250–700 mg L-cystine per day for general supplementation • 700 mg L-cystine + 280 mg L-theanine daily — the dose used in the Miyagawa and Kurihara immune RCTs • Hair/nail combination products typically include 100–500 mg L-cystine per serving

2. Higher studied dose

Up to 700 mg/day in the published combination RCTs (each combined with 280 mg L-theanine). No good evidence for going higher.

3. Timing

Take with or between meals. For vaccine support, start 4 days before vaccination and continue for 10–14 days. For training-block immune support, take throughout the heavy-training period.

4. With food

Either; with food may reduce any sulfur-related aftertaste.

5. Split dosing

Single morning dose is fine at 250–700 mg/day. No reason to split for absorption.

6. How long to try

Short courses (10–14 days) for vaccine or training-block support. Continuous use for hair/nail purposes hasn't been validated in standalone L-cystine trials.

What to track

Cold or upper-respiratory symptoms during the supplementation window

Subjective fatigue and recovery during heavy training

Hair / nail changes after several months (slow turnover means earlier assessments aren't meaningful)

Any kidney stone symptoms — relevant only for people with personal or family history

GI tolerance — most people have none, occasional bloating

Bottom line: If you're using it for the validated immune indications, mirror the trial protocol: 700 mg cystine + 280 mg theanine daily for 10–14 days bracketing vaccination or training stress.

4 commercial forms

Compare the main delivery options and what they’re best suited for.

L-cystine (free dimer)

Stable form

The disulfide-dimer form. Stable solid (free L-cysteine oxidises easily). Reduced to two L-cysteine molecules in the gut and liver.

Reliable cysteine delivery via reduction in vivo.

L-cystine + L-theanine combination

Trial-validated

The form used in the Miyagawa 2013 and Kurihara 2010 RCTs for immune support. Typical ratio 700 mg cystine : 280 mg theanine, taken daily for 10–14 days bracketing the immune challenge (vaccine or heavy training).

Same cystine pharmacokinetics; theanine adds CNS calming effects.

N-acetylcysteine (NAC) — sibling form

More clinical evidence

The acetylated form of L-cysteine. Far broader human evidence base — acetaminophen-overdose antidote, COPD mucolytic, fertility, mood, contrast-induced nephropathy prophylaxis. If your goal is glutathione precursor delivery, NAC is the better-validated choice for most indications.

More efficient cysteine delivery than L-cystine on a per-gram basis.

Whey protein (dietary cysteine source)

Food-based

Naturally rich in cysteine (especially un-denatured whey). For raising glutathione status modestly, daily whey protein is a low-cost food-based option supported by several small trials.

Whole-food protein; cysteine comes packaged with other essential amino acids.

Safety

Know the common side effects, key cautions, and who should avoid it.

Common side effects

mild GI upsetsulfur-related aftertaste or breath (uncommon at typical doses)

Serious risks

Cystinuria — a rare autosomal-recessive defect of cystine reabsorption in the kidney — leads to recurrent cystine kidney stones. Supplemental L-cystine should be strictly avoided in this condition.
SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements
High-dose chronic amino-acid supplementation can theoretically stress the kidneys in advanced chronic kidney disease; discuss with a nephrologist.
SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

Who should avoid it

Anyone with cystinuria or recurrent cystine kidney stones.
SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements
People with advanced chronic kidney disease — discuss supplemental amino-acid load with your nephrologist.
SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements
Pregnant or breastfeeding individuals using supplemental cystine beyond dietary intake — safety data are limited.
SourceMiyagawa et al., 2013 — Geriatrics & Gerontology International

Pregnancy & breastfeeding

Cystine and cysteine are routine dietary amino acids and intake from food poses no concern. Supplemental L-cystine specifically during pregnancy and breastfeeding has not been well studied — keep to dietary protein sources unless your clinician recommends otherwise.

SourceMiyagawa et al., 2013 — Geriatrics & Gerontology International

Bottom line: Generally well tolerated. The only concrete contraindication is cystinuria.

Interactions

Chemotherapy agents (cisplatin, doxorubicin, alkylators)Moderate

Antioxidant cysteine donors might theoretically reduce chemotherapy efficacy via glutathione-mediated drug detoxification. Discuss any cysteine supplementation with the treating oncologist.

SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

L-theanineMinor

Not an interaction in the safety sense — the L-cystine + L-theanine combination IS the form most modern human RCTs studied. Taking them together is the validated protocol.

SourceMiyagawa et al., 2013 — Geriatrics & Gerontology International Kurihara et al., 2010 — Journal of Amino Acids

N-acetylcysteine (NAC)Minor

Both feed the same cysteine pool. Combining is not unsafe but is rarely necessary; NAC alone is the more efficient cysteine donor for most clinical uses.

SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

Nitroglycerin and other nitratesMinor

Cysteine and its donors can theoretically restore nitrate sensitivity in nitrate-tolerant patients; clinical relevance for standalone L-cystine is uncertain.

SourceNIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements

Food sources

Food	Amount	%DV
Chicken breast, roasted	3 oz (~370 mg cystine + cysteine)	—
Beef, lean cooked	3 oz (~340 mg cystine + cysteine)	—
Pork loin, cooked	3 oz (~330 mg cystine + cysteine)	—
Eggs, large	2 eggs (~290 mg cystine + cysteine)	—
Tuna, canned	3 oz (~290 mg cystine + cysteine)	—
Whey protein isolate	1 scoop, ~25 g protein (~600 mg cystine + cysteine)	—
Lentils, cooked	1 cup (~230 mg cystine + cysteine)	—
Sunflower seeds	1 oz (~120 mg cystine + cysteine)	—
Oats, cooked	1 cup (~135 mg cystine + cysteine)	—

Chicken breast, roasted

Amount: 3 oz (~370 mg cystine + cysteine)
%DV: —

Beef, lean cooked

Amount: 3 oz (~340 mg cystine + cysteine)
%DV: —

Pork loin, cooked

Amount: 3 oz (~330 mg cystine + cysteine)
%DV: —

Eggs, large

Amount: 2 eggs (~290 mg cystine + cysteine)
%DV: —

Tuna, canned

Amount: 3 oz (~290 mg cystine + cysteine)
%DV: —

Whey protein isolate

Amount: 1 scoop, ~25 g protein (~600 mg cystine + cysteine)
%DV: —

Lentils, cooked

Amount: 1 cup (~230 mg cystine + cysteine)
%DV: —

Sunflower seeds

Amount: 1 oz (~120 mg cystine + cysteine)
%DV: —

Oats, cooked

Amount: 1 cup (~135 mg cystine + cysteine)
%DV: —

Choosing a product

What to look for on the label — and what to be skeptical of.

Look for…

Single-ingredient L-cystine OR explicit L-cystine + L-theanine combination (the form studied in modern RCTs)

L-cystine dose disclosed in milligrams per serving (700 mg matches the trial dosing)

Pharmaceutical or USP-grade amino acid

Third-party identity / purity testing

Capsules over loose powder (L-cystine is bitter and sulfur-tinged)

Be skeptical of…

'Detox' or 'liver cleanse' framing — claims more honestly belong on NAC's evidence base, not standalone L-cystine

'Anti-aging' or 'glutathione booster' marketing without trial citations — most glutathione data use NAC or whey protein

Hair-regrowth claims for androgenetic or hormonal hair loss — neither L-cystine nor any amino acid reverses these

Combination products that obscure the L-cystine dose per serving

Megadose products (>1500 mg/day) — there's no human evidence supporting them and they add unnecessary amino-acid load

Frequently asked questions

Is L-cystine the same as cysteine?⌄

Cystine is two cysteine molecules joined by a disulfide bond. The body reduces it to cysteine after ingestion.

Should I take cystine or NAC?⌄

NAC has more clinical evidence. Cystine is a reasonable alternative if NAC is unavailable.

References by claim

Influenza vaccine response in older adults

Miyagawa et al., 2013 — Geriatrics & Gerontology International (2013) link

Common cold prevention in stressed athletes

Kurihara et al., 2010 — Journal of Amino Acids (2010) link

Post-surgical inflammation and infection

Murakami et al., 2009 — Surgery Today (2009) link

Hair, skin, and nails

NIH ODS information on cysteine and glutathione — NIH Office of Dietary Supplements link

Other references

L-Cystine on PubChem (CID 595) — PubChem link

L-Cystine (ChEBI:16283) — ChEBI link

L-Cystine on Wikidata — Wikidata link

Track L-Cystine with Pilora

Set up dose reminders, check interactions, and join the community in the Pilora iPhone app.

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Evidence-based·Last reviewed May 31, 2026·Evidence current as of May 31, 2026·How we grade evidence

Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you’re pregnant, breastfeeding, on medications, or managing a chronic condition.