Cysteine

amino acid

What is it

Cysteine is a conditionally essential sulfur-containing amino acid used in protein synthesis and as the precursor to glutathione, the body's major intracellular antioxidant. It can be synthesized from methionine but dietary intake becomes important during illness or restricted protein intake.

How it works

Cysteine is absorbed in the small intestine and used in protein synthesis, where its sulfur-containing thiol (-SH) group forms disulfide bonds that determine protein folding and structural integrity. Keratin (hair, nails) is rich in cysteine and depends on these disulfide bonds for strength. The most important metabolic role of cysteine is providing the rate-limiting building block for glutathione, the tripeptide that serves as the body's main intracellular antioxidant. Glutathione neutralizes reactive oxygen species, supports Phase II detoxification through conjugation, and recycles vitamins C and E. Adequate cysteine supply directly determines how much glutathione cells can produce. Cysteine is classified as conditionally essential because the body can synthesize it from methionine through the transsulfuration pathway, but this pathway requires vitamins B6, B12, and folate, and may not meet demand during illness, oxidative stress, or restricted methionine intake.

Evidence for 5 uses

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

Acetaminophen overdose (as NAC)

Grade A

Strong evidence

NAC is the established antidote for acetaminophen poisoning, providing cysteine for glutathione synthesis to detoxify the toxic metabolite. Standard emergency intervention.

Glutathione synthesis support

Grade B

Good evidence

Cysteine is the rate-limiting amino acid for glutathione production. Supplementation (especially as NAC) reliably raises intracellular glutathione and supports antioxidant defenses.

Mucolytic for respiratory conditions (NAC)

Grade B

Good evidence

NAC at 600-1,200 mg/day thins respiratory mucus and helps clearance in chronic bronchitis, COPD, and cystic fibrosis.

Polycystic ovary syndrome (PCOS, as NAC)

Grade B

Good evidence

NAC at 1.2-1.8 g/day has reasonable evidence for improving insulin sensitivity and ovulation in PCOS, comparable to metformin in some trials.

Hair and nail health

Grade C

Moderate evidence

Cysteine is highly concentrated in keratin. Combination supplements (cysteine, biotin, B vitamins, zinc) for hair are common. Evidence for cysteine alone is limited.

4 commercial forms

L-cysteine (free form)

Less stable than NAC; oxidizes readily.

Used in some hair, skin, and nail formulations. NAC is generally preferred for systemic effects.

N-acetylcysteine (NAC)

More stable, better absorbed; the standard clinical and supplement form.

The form with the most clinical evidence for glutathione support, respiratory health, and other indications.

L-cysteine HCl

Hydrochloride salt; slightly more stable than free form.

Used in some supplement formulations.

L-cystine

Oxidized dimer; reduced to cysteine after absorption.

Less commonly used than NAC; found in some hair-targeted products.

Dosage

There is no separate RDA for cysteine; combined with methionine, the requirement is 19 mg/kg/day. Supplement doses of plain cysteine range 500-1,500 mg/day. N-acetylcysteine (NAC), the more common and stable form, is dosed at 600-1,800 mg/day for most uses. No formal UL.

When and how to take it

Cysteine can be taken with or without food. Empty stomach may improve absorption but increase odor. Split larger doses across the day. NAC is more practical for most uses due to better stability and shelf life.

Food sources

Food	Amount	%DV
Chicken breast (3 oz, cooked)	330 mg	—
Beef (3 oz, cooked)	260 mg	—
Tuna (3 oz, cooked)	300 mg	—
Eggs (1 large)	150 mg	—
Yogurt (1 cup)	100 mg	—
Sunflower seeds (1 oz)	130 mg	—
Oats (1 cup, cooked)	100 mg	—
Lentils (1 cup, cooked)	230 mg	—

Safety

Cysteine at typical doses is generally well-tolerated. Higher doses can cause GI upset, headache, and a sulfur odor. Long-term high-dose safety is less well-characterized than NAC. People with cystinuria must avoid supplementation due to risk of cystine kidney stones.

Who should be cautious

Avoid in cystinuria. Caution in pregnancy and lactation. Patients on certain chemotherapy regimens should consult oncologist. People undergoing treatment for HIV with certain protease inhibitors should be aware of potential interactions.

Interactions

May modestly enhance nitrate medications via thiol donation. May interfere with certain oxidative-stress-based chemotherapies. Adequate B-vitamin status supports cysteine synthesis from methionine. Few other significant drug interactions documented.

Frequently asked questions

Should I take cysteine or NAC?⌄

NAC is generally preferred because it is more stable, better absorbed, and has more clinical evidence. Plain L-cysteine has its uses but for most systemic purposes NAC is the better choice.

Will cysteine raise glutathione?⌄

Yes, particularly NAC. Cysteine availability is the rate-limiting step for glutathione synthesis. Multiple studies show NAC raises tissue and plasma glutathione levels.

Is cysteine good for hair?⌄

Cysteine is concentrated in keratin and disulfide bonds give hair its structure. Supplementation may help in some individuals, but evidence for cysteine alone is limited; combination products with B vitamins, zinc, and biotin are more commonly studied.

Why does my cysteine supplement smell?⌄

Cysteine is sulfur-containing, and small amounts of sulfur compounds can produce characteristic odors. NAC tends to have less odor than plain L-cysteine.

Can I take cysteine with chemotherapy?⌄

Discuss with your oncologist. Some chemotherapy agents rely on oxidative stress, and high-dose cysteine or NAC could theoretically interfere. The interaction depends on the specific regimen.

References

Cysteine - Wikidata — Wikidata link

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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.