Also known as: N-Acetylcysteine, NAC, Acetylcysteine, Cysteine precursor
FoundationalGood evidence
The acetylated form of cysteine, and the most practical way to raise glutathione. Cysteine availability is what limits glutathione production, which is why supplying it directly works better than taking glutathione itself.
Typical dose
600.0–1800.0 mg per day
With food; away from training sessions if training for adaptation
What it is
N-acetylcysteine is cysteine with an acetyl group attached, which improves stability and absorption compared with free cysteine. Cysteine itself is a sulphur-containing amino acid obtained from protein foods, particularly eggs, poultry and dairy.
Its significance rests on being the rate-limiting precursor for glutathione. Glutathione synthesis requires cysteine, glutamate and glycine, and cysteine availability is what constrains the process. Supplying it directly is a more efficient route to raising glutathione than taking glutathione itself.
NAC has a long history of supervised medical use going back decades, which is unusual for something sold as a supplement and means its pharmacology is better characterised than most ingredients in this index.
How it works
Once absorbed, NAC is deacetylated to cysteine, which enters the glutathione synthesis pathway. Because cysteine is the limiting substrate, raising its availability directly increases glutathione production capacity, particularly in the liver.
NAC also has a direct chemical action on mucus. Its free thiol group breaks the bonds that make mucus thick and sticky, reducing its viscosity. This is a physical effect rather than a metabolic one.
It additionally modulates glutamate levels in the brain through the cystine-glutamate antiporter, which is the basis for a strand of research quite separate from its antioxidant role.
What the evidence shows
NAC has a long record of supervised medical use, which is unusual provenance for a supplement ingredient and gives reasonable confidence that it does something.
For raising glutathione, NAC is well supported and is generally the more efficient and cheaper route compared with oral glutathione. Research in exercise contexts is mixed, with some trials reporting reduced fatigue markers and others finding that high-dose antioxidant use blunts training adaptations.
There is a broader research literature in supervised medical settings that sits outside the scope of supplement use. Anyone considering NAC for a specific reason should be doing so with medical guidance rather than self-directed.
Dosing
Common supplemental range is 600-1,800 mg daily, often split into two servings. 600 mg once or twice daily is the typical general-purpose dose.
Taking it on an empty stomach improves absorption, though it is more likely to cause nausea that way, so with food is a reasonable trade for most people.
If training for adaptation, be aware that high-dose antioxidant supplementation around sessions has been shown to blunt some exercise adaptations. Taking it away from training, or reserving it for rest days, avoids that concern. Combining with glycine, as GlyNAC, targets two of the three glutathione precursors.
With food; away from training sessions if training for adaptation
Competition status
Permitted — not on the WADA Prohibited List
Category
Foundational
Safety and side effects
NAC is well tolerated at supplemental doses with a long record of use. Digestive effects are the most common complaint: nausea, vomiting and diarrhoea, generally dose-related and reduced by taking with food.
It has a distinctly sulphurous smell and taste that many people find unpleasant, which is worth knowing before buying powder rather than capsules.
Because it affects platelet function and has a mild widening effect on blood vessels, check with your doctor before use if you take any regular medication, particularly anything affecting blood clotting or blood pressure.
Who should avoid it
Speak to your doctor before use if you take any regular medication, particularly anything affecting blood clotting or blood pressure. Avoid taking alongside activated charcoal, which reduces absorption. Not established for use in pregnancy or breastfeeding at supplemental doses.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
