Comparison guide
NAC or L-cysteine?
Cysteine limits glutathione synthesis, but almost all the clinical research uses N-acetylcysteine rather than free L-cysteine, which is unstable and largely oxidises to cystine. The one niche use for plain cysteine is a slow-release lozenge for smoking cessation.
The options, compared
| Option | Evidence | Best for | The honest note | |
|---|---|---|---|---|
| NAC (N-Acetylcysteine) | Moderate evidence | Most people | The form behind the glutathione and COPD research, with 1200 mg a day reducing exacerbations more consistently than 600 mg. The psychiatric and general-wellness claims have largely failed, and its US supplement status remains legally unsettled. | View |
| L-Cysteine | Limited evidence | Smoking-cessation lozenges only | Two manufacturer-linked trials of slow-release lozenges providing 25 mg found higher quit rates, but 500 mg capsules have no glutathione data of their own. People with cystinuria should avoid it. | View |
Our verdict
NAC is the default: it is the form used in essentially all the research and is more stable in a capsule, while plain L-cysteine offers no practical advantage. Take NAC for a specific reason such as COPD rather than general wellness, and do not combine it with nitroglycerin.