N-Acetylcysteine and Sports Performance: Is There an Effect?

N-acetylcysteine, better known as NAC, has a long medical history as an antidote to paracetamol poisoning and as a mucolytic. In sport it is discussed as an antioxidant that might delay fatigue and protect organs from oxidative stress. The research is genuinely interesting but far from a simple "yes". Our editorial team looks at what has been tested and what it means for training.

What NAC does in the body

NAC is an acetylated form of the amino acid cysteine. Once absorbed, it supplies cysteine, the rate-limiting building block for glutathione, the body's principal intracellular antioxidant. This is the basis of its medical use in paracetamol overdose, where glutathione stores in the liver are depleted.

NAC can also directly neutralise some reactive oxygen species and break disulphide bonds in mucus, which explains its use in some respiratory conditions. It is studied in psychiatry, too, because of effects on glutamate signalling.

During intense or prolonged exercise, muscles produce more reactive oxygen and nitrogen species. At moderate levels these act as signals that drive adaptations such as mitochondrial growth; at high levels they may contribute to fatigue and muscle damage.

The theoretical case for NAC in sport therefore rests on buffering excessive oxidative stress to sustain muscle function. The theoretical case against it is that it may also blunt useful signals.

Evidence for delaying fatigue

An early study by Reid and colleagues found that NAC given intravenously reduced fatigue during repetitive electrical stimulation of a leg muscle. It established that redox balance plays a role in muscle fatigue in humans.

Later, Medved and co-workers infused NAC into endurance-trained cyclists during prolonged exercise and showed increased muscle cysteine and glutathione availability, along with improved time to fatigue. McKenna and colleagues reported that NAC attenuated the decline in muscle sodium-potassium pump activity and delayed fatigue.

These findings were obtained with intravenous infusion at high, weight-based doses under laboratory supervision. Such protocols caused side effects in some participants and are not comparable to taking oral supplements.

Studies of oral NAC have produced mixed results. Paschalis and colleagues offered an important insight: after 30 days of oral supplementation at 1200 mg per day, performance and oxidative stress markers improved only in individuals with low baseline glutathione, with little or no benefit in those with normal levels.

Recovery and training adaptation

NAC's effect on recovery from muscle-damaging exercise is double-edged. Michailidis and colleagues found that NAC reduced some markers of inflammation after intense eccentric exercise but also altered muscle signalling pathways and, in some measures, delayed recovery of strength.

This fits a broader concern in sports science. Ristow and co-workers showed that high-dose vitamins C and E prevented some of the health-promoting effects of exercise on insulin sensitivity, by suppressing the oxidative signals that trigger adaptation.

The practical implication is that chronic, high-dose antioxidant use during a training block may be counterproductive for people seeking long-term gains. Short-term use around competition, when performance rather than adaptation is the goal, is a different situation.

Researchers increasingly argue for a personalised approach, in which antioxidants are considered only for those with demonstrable deficits, rather than as a universal supplement.

Safety, status and practical points

In the UK, NAC is used as a hospital medicine for paracetamol poisoning and is also sold as a food supplement, typically in 600 mg capsules. Common side effects of oral NAC include nausea, bloating, diarrhoea and an unpleasant sulphurous smell.

Points to consider before using NAC:

• people with asthma should be cautious, as bronchospasm has been reported, mainly with inhaled or intravenous forms;
• NAC may enhance the effects of nitroglycerin, causing headaches and low blood pressure;
• those with peptic ulcers or bleeding disorders, and pregnant women, should seek medical advice first;
• it is not on the WADA prohibited list, but supplements should be batch-tested for contamination.

NAC is sometimes promoted as "liver protection" for people using hepatotoxic substances. Its role in paracetamol poisoning does not translate into protection against anabolic steroid or other drug-induced liver injury, and it should not be relied on as such.

If you suspect low antioxidant status, a balanced diet with adequate protein, fruit and vegetables supports glutathione production naturally, as cysteine is abundant in protein-rich foods.

Conclusions of the editorial team

NAC can delay fatigue under laboratory conditions, especially when given intravenously, and oral NAC may help people with low glutathione levels. For most well-nourished athletes, the measurable benefit of oral supplements is small.

Chronic high-dose use carries the theoretical cost of blunting training adaptations, a concern supported by research on other antioxidants. Short, targeted use makes more sense than daily long-term supplementation.

NAC is not a safety net for hepatotoxic drugs, and anyone with asthma, ulcers or on nitrate medicines should consult a doctor before use.

You might also find our articles on antioxidants and training adaptation, glutathione testing and liver enzymes after training useful.

References

1. Reid MB, Stokić DS, Koch SM, Khawli FA, Leis AA. N-acetylcysteine inhibits muscle fatigue in humans. J Clin Invest. 1994;94(6):2468–2474.
2. Medved I, Brown MJ, Bjorksten AR, et al. N-acetylcysteine enhances muscle cysteine and glutathione availability and attenuates fatigue during prolonged exercise in endurance-trained individuals. J Appl Physiol. 2004;97(4):1477–1485.
3. McKenna MJ, Medved I, Goodman CA, et al. N-acetylcysteine attenuates the decline in muscle Na+,K+-pump activity and delays fatigue during prolonged exercise in humans. J Physiol. 2006;576(Pt 1):279–288.
4. Paschalis V, Theodorou AA, Margaritelis NV, Kyparos A, Nikolaidis MG. N-acetylcysteine supplementation increases exercise performance and reduces oxidative stress only in individuals with low levels of glutathione. Free Radic Biol Med. 2018;115:288–297.
5. Michailidis Y, Karagounis LG, Terzis G, et al. Thiol-based antioxidant supplementation alters human skeletal muscle signaling and attenuates its inflammatory response and recovery after intense eccentric exercise. Am J Clin Nutr. 2013;98(1):233–245.
6. Ristow M, Zarse K, Oberbach A, et al. Antioxidants prevent health-promoting effects of physical exercise in humans. Proc Natl Acad Sci U S A. 2009;106(21):8665–8670.
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