NAC, or N-acetylcysteine, is a modified form of cysteine, a sulphur-containing amino acid. It has a dual role: a medicine in hospitals and pharmacies, and an ingredient in food supplements. Many effects are attributed to it, often summed up as “detox”. This article separates the evidence: what NAC is, its medical uses, what trials actually show, studied doses and precautions.
Key takeaways
- NAC provides cysteine, one of the three amino acids making up glutathione.
- In medicine, it is used as an antidote to paracetamol and as a mucolytic. These uses do not apply to a food supplement.
- The strongest trials concern chronic bronchitis and COPD. Elsewhere, results are preliminary or conflicting.
- Only 6 to 10% of a swallowed dose reaches the blood intact. It is generally well tolerated.
What is NAC?
N-acetylcysteine is cysteine with an added acetyl group. Like cysteine, it carries a thiol group, containing a highly reactive sulphur atom that explains most of its activity.
Cysteine itself occurs in protein-rich foods: poultry, eggs, dairy, sunflower seeds and pulses. The body can also make it from another amino acid, methionine. Vitamin B6 contributes to normal cysteine synthesis, as explained in our article on vitamin B6. Cysteine is also used to produce taurine, another sulphur-containing compound.
How does NAC work?
Once absorbed, NAC releases cysteine. Cysteine is one of the three building blocks of glutathione, alongside glutamate and glycine. Glutathione is the main antioxidant molecule made by our cells. When its stores collapse, as in paracetamol poisoning, NAC helps replenish them.
This mechanism has an important limitation. A 2014 review stresses that NAC's antioxidant effect requires pre-existing glutathione deficiency and functioning synthesis pathways. In someone with normal stores, an additional intake has not been shown to change anything. NAC also acts on mucus: its thiol group breaks chemical bridges that stiffen bronchial secretions.
What is NAC used for in medicine?
These uses are presented for context. They concern medicines used at specific doses and within a defined medical setting, and do not apply to a food supplement.
The antidote for paracetamol poisoning
At high doses, paracetamol depletes liver glutathione and produces a toxic metabolite. NAC is the standard treatment. In a large US study involving 2,540 patients, liver injury occurred in 6.1% of at-risk patients when NAC started within ten hours, versus 26.4% when started between ten and 24 hours. Given within eight hours, it was protective regardless of initial blood paracetamol concentration, highlighting the urgency of seeking care. The 2018 Cochrane review judged randomised trials rare and low-quality, but recommended NAC for everyone at risk.
A mucolytic
NAC is also used to thin bronchial secretions. For this use, the maximum authorised dose for prolonged treatment is 600mg daily, as noted by a 2021 safety review.
What do clinical trials show?
Results below come from research trials, often in patients. They describe what was measured, not an effect that can be attributed to a supplement.
Airways and winter infections
A meta-analysis of 13 studies involving 4,155 patients with chronic bronchitis or COPD observed 25% fewer exacerbations with NAC. When airway obstruction was confirmed, authors recommended at least 1,200mg daily. The largest trial, conducted in China in 1,006 COPD patients, tested 600mg twice daily for one year. The result: 1.16 exacerbations per patient per year versus 1.49 with placebo. This trial was funded by a NAC manufacturer.
Winter infections have also been studied. In 262 mainly older people, 600mg twice daily for six months did not prevent influenza infection. However, only 25% of infected participants had symptoms with NAC, versus 79% with placebo. This result from a 1997 trial requires confirmation.
Compulsive behaviours and addictions
NAC acts on glutamate, a brain chemical messenger involved in compulsions. In a twelve-week trial in 50 adults with trichotillomania, compulsive hair-pulling, 56% taking NAC (1,200 to 2,400mg daily) were clearly improved versus 16% on placebo. In 116 adolescents dependent on cannabis, it more than doubled the chance of negative urine tests. But in 302 adults, at the same dose of 1,200mg twice daily, it had no effect. These disorders require medical care.
Is NAC a “detox” ingredient?
Not in the marketing sense. The liver continuously eliminates foreign substances, notably using glutathione. NAC helps replenish glutathione when depleted, as in paracetamol poisoning, and this antidote role fuelled its reputation. There is no evidence that it “cleanses” a healthy person's body. No health claim is authorised for NAC in the European Union.
What happens to NAC after swallowing?
It has low bioavailability. In ten volunteers taking 600mg in different forms, only 6 to 10% of the dose reached the circulation intact. Its elimination half-life is short, just over two hours.
What dose of NAC?
Trials used 600 to 2,400mg daily, most often in two doses of 600 to 1,200mg. The table summarises the main studied protocols.
| Context | Studied dose | Duration | Result |
|---|---|---|---|
| Paracetamol (hospital) | 140mg/kg, then 70mg/kg every 4 h | 72 h | Liver injury: 6.1% if started before 10 h |
| COPD | 600mg twice daily | 1 year | 1.16 versus 1.49 exacerbations per year |
| Influenza (older people) | 600mg twice daily | 6 months | Symptomatic cases: 25% versus 79% |
| Trichotillomania | 1,200 to 2,400mg daily | 12 weeks | 56% improved versus 16% |
| Cannabis (adults) | 1,200mg twice daily | 12 weeks | No effect |
NAC is available in capsules, tablets and sachets. For these different dosage forms, see our dedicated article.
Adverse effects and precautions
Oral NAC is generally well tolerated. A review of 41 studies in respiratory patients at doses of 600 to 3,000mg daily concluded that digestive symptoms were no more frequent than in control groups. The most commonly reported adverse effects remain digestive.
- Nitrates: in 11 volunteers, a very high NAC dose (100mg/kg) intensified nitroglycerin-induced headaches and arterial dilation. Mention NAC if you are treated for angina.
- Cancer: in mice, NAC accelerated lung cancer progression. This animal result has not been confirmed in humans, but warrants oncologist advice in cancer or high-risk situations, such as smoking.
- Pregnancy, breastfeeding and children: supplemental NAC has not been evaluated in these situations. Seek medical advice.
Frequently asked questions
When should NAC be taken: morning or evening?
No trial has compared intake times. In most studies, the daily dose was divided into morning and evening doses. The most important point is following the stated dose.
How long can NAC be taken?
Daily use for one year at 1,200mg was well tolerated in COPD trials. Beyond this, data are lacking. Prolonged use, especially at high doses, should be discussed with a healthcare professional.
NAC or glutathione: which should you choose?
They are not equivalent. NAC supplies the cysteine cells need to make their own glutathione. Swallowed glutathione raises an absorption question discussed in our article on glutathione.
Is NAC good for the liver?
Its only established liver use is as a paracetamol antidote in hospital. The NIH LiverTox database does not associate it with liver injury, but no trial shows that it improves a healthy liver. Plants traditionally associated with the liver are presented in our article on milk thistle.
Is NAC a medicine?
It exists in both categories. In France, acetylcysteine is a mucolytic medicine sold in pharmacies and an antidote used in hospitals. It is also sold as a food supplement, such as Hexagon® NAC 600mg (600mg per capsule). Only a medicine may claim a therapeutic effect.
Why does NAC smell like rotten eggs?
Its thiol group contains sulphur, like the compounds responsible for the smell of rotten eggs. A slight sulphurous odour is therefore usual with this molecule and is not in itself concerning.
Conclusion
NAC is an established medical molecule: the standard paracetamol antidote and a mucolytic. Beyond those uses, the strongest trials concern COPD, with fewer exacerbations at 1,200mg daily. Psychiatric possibilities remain promising but uneven, and “detox” NAC for a healthy person has no supporting evidence. Poorly absorbed but well tolerated, NAC is not harmless for everyone: discuss it with your doctor if you take medication or have a history of cancer.
Sources
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- Chiew AL et al. (2018). Interventions for paracetamol (acetaminophen) overdose. Cochrane Database of Systematic Reviews, 2, CD003328. doi:10.1002/14651858.CD003328.pub3
- Rushworth GF, Megson IL (2014). Existing and potential therapeutic uses for N-acetylcysteine: the need for conversion to intracellular glutathione for antioxidant benefits. Pharmacology & Therapeutics, 141(2), 150-159. doi:10.1016/j.pharmthera.2013.09.006
- Calverley P, Rogliani P, Papi A (2021). Safety of N-Acetylcysteine at High Doses in Chronic Respiratory Diseases: A Review. Drug Safety, 44(3), 273-290. doi:10.1007/s40264-020-01026-y
- Cazzola M et al. (2015). Influence of N-acetylcysteine on chronic bronchitis or COPD exacerbations: a meta-analysis. European Respiratory Review, 24(137), 451-461. doi:10.1183/16000617.00002215
- Zheng JP et al. (2014). Twice daily N-acetylcysteine 600mg for exacerbations of chronic obstructive pulmonary disease (PANTHEON): a randomised, double-blind placebo-controlled trial. The Lancet Respiratory Medicine, 2(3), 187-194. doi:10.1016/S2213-2600(13)70286-8
- De Flora S, Grassi C, Carati L (1997). Attenuation of influenza-like symptomatology and improvement of cell-mediated immunity with long-term N-acetylcysteine treatment. European Respiratory Journal, 10(7), 1535-1541. doi:10.1183/09031936.97.10071535
- Grant JE, Odlaug BL, Kim SW (2009). N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study. Archives of General Psychiatry, 66(7), 756-763. doi:10.1001/archgenpsychiatry.2009.60
- Gray KM et al. (2012). A double-blind randomized controlled trial of N-acetylcysteine in cannabis-dependent adolescents. American Journal of Psychiatry, 169(8), 805-812. doi:10.1176/appi.ajp.2012.12010055
- Gray KM et al. (2017). A randomized placebo-controlled trial of N-acetylcysteine for cannabis use disorder in adults. Drug and Alcohol Dependence, 177, 249-257. doi:10.1016/j.drugalcdep.2017.04.020
- Borgström L, Kågedal B, Paulsen O (1986). Pharmacokinetics of N-acetylcysteine in man. European Journal of Clinical Pharmacology, 31(2), 217-222. doi:10.1007/BF00606662
- Iversen HK (1992). N-acetylcysteine enhances nitroglycerin-induced headache and cranial arterial responses. Clinical Pharmacology & Therapeutics, 52(2), 125-133. doi:10.1038/clpt.1992.121
- Sayin VI et al. (2014). Antioxidants accelerate lung cancer progression in mice. Science Translational Medicine, 6(221), 221ra15. doi:10.1126/scitranslmed.3007653
- LiverTox (regularly updated). Acetylcysteine. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, National Institute of Diabetes and Digestive and Kidney Diseases. PubMed 31643720
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
