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Limited evidenceGeneral health

L-Cysteine

The sulfur amino acid that limits glutathione synthesis. Almost all clinical research uses the N-acetylcysteine form instead, and the trials of plain L-cysteine are about binding acetaldehyde in the mouth and stomach.

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What the science says

  • Slow-release lozenges and smoking cessation Moderate evidence

    A meta-analysis of two randomised, double-blind, placebo-controlled trials of slow-release L-cysteine lozenges found higher smoking abstinence rates than placebo, the proposed mechanism being that cysteine binds the acetaldehyde in tobacco smoke. Both trials come from the same group and involve the product's manufacturer, which limits how much weight the result can carry.

  • Glutathione precursor Limited evidence

    Cysteine availability is the rate-limiting step for glutathione synthesis, but the human trials that demonstrate raised glutathione use N-acetylcysteine or glycine plus NAC, not free L-cysteine. Free cysteine is poorly stable and largely oxidises to cystine, so the extrapolation from NAC data is not safe to make.

Dosage & safety

Studied doseThe smoking-cessation trials used slow-release lozenges providing 25 mg of L-cysteine, taken repeatedly through the day. Capsule products typically supply 500 mg; for glutathione support, N-acetylcysteine at 600–1800 mg/day is the form that has actually been studied.
SafetyGenerally tolerated at supplement doses. Gram-scale free cysteine can cause nausea and, in theory, raise homocysteine-pathway load; people with cystinuria should avoid it. It is not a substitute for NAC in paracetamol poisoning, which is a hospital treatment. No pregnancy data.

How to take it

TimingThe smoking-cessation lozenges were used repeatedly through the day; for capsules, timing does not matter.
With food?Take with food, since free cysteine at gram doses commonly causes nausea.
Worth knowingIf your goal is glutathione, take N-acetylcysteine instead — that is the form actually studied, and it is more stable than free cysteine.

Frequently asked questions

Is L-cysteine the same as NAC?

No. N-acetylcysteine is the form used in almost all clinical research and the one shown to raise glutathione; free L-cysteine is poorly stable and largely oxidises to cystine. If your goal is glutathione, NAC at 600–1800 mg/day is the studied option.

Does L-cysteine help you quit smoking?

Possibly: a meta-analysis of two placebo-controlled trials of slow-release L-cysteine lozenges found higher abstinence rates, with moderate evidence, but both trials involve the product's manufacturer. The lozenges provided 25 mg of L-cysteine, taken repeatedly through the day.

Does L-cysteine have side effects?

L-cysteine is generally tolerated at supplement doses, but gram-scale free cysteine can cause nausea, so take it with food. People with cystinuria should avoid L-cysteine, and there are no pregnancy data.

Scientific references

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