Supplement analysis backed by scientific evidence · PT

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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.

Evidence last reviewed: August 2026

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.

Scientific references

Where to buy

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