What Is NAC (N-Acetylcysteine)? Uses, Evidence and Safety in the UK

NAC (N-acetylcysteine) is a form of the amino acid cysteine. In the UK, acetylcysteine is a licensed medicine: the hospital antidote to paracetamol overdose, a mucolytic taken by mouth to make sputum less sticky, and an eye drop.¹ NAC is also sold as a food supplement with no authorised health claim.² It belongs beside the Wellgard glutathione guide because cysteine usually limits how much glutathione a cell can make.

Evidence at a glance

• Trials: four pooled analyses and 16 controlled trials of 24 to 4,993 people.
• Largest trial: 4,993 people before angiography; five days of oral NAC made no difference to kidney outcomes at 90 days.³
• Strongest measured outcome: in 28 trials of mucolytics, NAC among them, 6,723 people with COPD or chronic bronchitis, more stayed free of flare-ups than on placebo (moderate certainty, the review's grading).⁴
• Not tested in any trial we found: NAC by mouth in healthy adults for skin, urinary infections, weight or cortisol.

Sources checked on 13 October 2026.

What is NAC, and is it the same as acetylcysteine?

Yes. N-acetylcysteine, N-acetyl-L-cysteine, acetylcysteine and NAC name one molecule: cysteine with an acetyl group attached to its nitrogen atom, the "N". Taken by mouth, 200 to 400 mg peaks in the blood within one to two hours, with a half-life of about six hours, and the main product leaving the body is sulphate.⁵

Is NAC a medicine or a supplement in the UK?

Both, and no health claim is authorised for it: the assessors refused the wording that it contributes to normal glutathione formation in 2010, and an "elimination of heavy metals" wording for a NAC combination in 2011.²

The British National Formulary lists three uses: by drip in hospital for paracetamol overdose, by mouth for "reduction of sputum viscosity", and as eye drops for tear deficiency.¹ For COPD, the NHS says that if carbocisteine does not help, "another mucolytic medicine called acetylcysteine is available".⁶ On paracetamol, the NHS says to get help from NHS 111 if "you've taken more paracetamol than it says on the packet or leaflet".⁷ Questions about the medicine go to a pharmacist or GP.

Across the EU, a 2024 Dutch government safety assessment records that "In many EU Member States NAC and L-cysteine have a so-called dual status": sold as a medicine and as a food supplement.⁸

What has NAC been tested for?

Mostly as a medicine, in people with a diagnosed condition, at 600 to 3,000 mg a day. Results run from fewer COPD flare-ups in some large trials to no difference in lung fibrosis, kidney protection, fatty liver, OCD and hangover.

Every row is a medical condition, and a question for your GP or specialist; NAC was given at medicine doses, usually alongside prescribed care. For mental health, NHS Talking Therapies can be reached directly or through a GP.

Does NAC raise glutathione?

NAC supplies cysteine, the amino acid that usually limits how much glutathione a cell can make; glutathione supplements supply the finished molecule. The two have different trial records and no trial has compared them on a health outcome. The claim that NAC contributes to normal glutathione formation was refused.²

In eight older adults, two weeks of cysteine, given as NAC, plus glycine brought glutathione synthesis back to the rate of eight younger adults; there was no placebo group.²⁴ A 16-week placebo-controlled trial of the pair (GlyNAC) in 24 older adults, from the same US group, found low glutathione and several markers of ageing improved.²⁵ The largest GlyNAC trial, 114 healthy adults averaging 65, found no rise in total glutathione after two weeks; only a post-hoc subgroup with high oxidative stress and low glutathione rose.²⁶ The two routes are compared on NAC vs glutathione.

What doses did the trials use?

Trials used 600 to 3,000 mg a day as medicines; the maximum licensed long-term dose is 600 mg a day. Ask a pharmacist, especially if you already take a NAC medicine.²⁷

A review of 41 studies in chronic lung disease, sponsored by a NAC medicine maker, found "the safety profile was similar at both the high and standard doses" by mouth.²⁷ The PCOS (now called PMOS by the NHS) trials used 1,200 to 1,800 mg a day.¹⁹ The Dutch assessment advises against taking a NAC supplement alongside a NAC medicine, "as people may take too much".⁸ Side effects, interactions and pregnancy are on NAC side effects.

What we don't know

• The placebo-controlled trials of NAC alone in healthy adults that we found lasted a month or less.^{21, 22, 23}
• We found no NAC trial that measured mood in healthy adults; the mental health trials enrolled people with a diagnosis, mostly alongside their treatment.^{14, 15, 16}
• The large COPD trials of NAC alone disagree, and later mucolytic trials found smaller effects than early ones.^{4, 9, 10, 11}
• The GlyNAC record is one US group's trials and one larger two-week trial that found no overall rise.^{25, 26}

Frequently asked questions

Is NAC good for the liver?

Its liver use is a hospital treatment by drip, as the antidote to paracetamol overdose.¹ By mouth, an eight-week trial in 69 people with fatty liver changed neither liver fat nor liver enzymes.¹³ No liver claim is authorised.²

Does NAC help PCOS?

NAC has been tested in women with polycystic ovary syndrome, mostly at 1,200 to 1,800 mg a day as a medicine alongside fertility treatment. A 2015 pooling of eight trials in 910 women found ovulation and pregnancy more likely than on placebo and less likely than on metformin; its only live-birth trial, 60 women, was retracted in 2025, and four of the 18 trials in a 2023 hormone pooling have since been retracted, so neither number is repeated here.^{19, 28, 29} PCOS is a GP diagnosis; the graded table is on which PCOS supplements have evidence.

Can NAC help a hangover?

Two double-blind trials, 49 and 40 adults, found no difference in hangover scores against placebo.^{22, 23} No supplement may claim anything about a hangover; the other ingredients tested are on hangover pills: what the trials found.

Is NAC found in food?

Food supplies cysteine rather than NAC, within protein from meat, fish, eggs, dairy and pulses; the NAC in medicines and supplements is manufactured. The foods richest in glutathione and its building blocks are on what foods are high in glutathione.

Does NAC help skin, UTIs, weight or cortisol?

We found no trial of NAC by mouth for these in healthy people, and none for UTIs or BV in women. The one placebo-controlled acne trial we found tested a skin gel, not capsules, in 99 people.³⁰

Sources

1. NICE. British National Formulary: acetylcysteine. bnf.nice.org.uk
2. Great Britain nutrition and health claims (NHC) register, 19 May 2026 edition. gov.uk
3. Weisbord et al. N Engl J Med 2018. Randomised trial, 4,993 people, 90 days. PubMed 29130810
4. Poole et al. Cochrane 2019. Pooled analysis of 38 trials, 10,377 people. PubMed 31107966
5. Holdiness MR. Clin Pharmacokinet 1991. Review of pharmacokinetic studies. PubMed 2029805
6. NHS. COPD: treatment. Page last reviewed 11 April 2023 (review overdue). nhs.uk
7. NHS. Paracetamol for adults. Page last reviewed 24 March 2026. nhs.uk
8. RIVM, the Netherlands public health institute. Letter report 2024-0086, 2024. Safety assessment. doi.org
9. Decramer et al. Lancet 2005. Randomised trial, 523 people, 3 years. PubMed 15866309
10. Zheng et al. Lancet Respir Med 2014. Randomised trial, 1,006 people, 1 year; manufacturer-funded. PubMed 24621680
11. Zhou et al. Nat Commun 2024. Randomised trial, 968 people, 2 years. PubMed 39349461
12. Martinez et al. N Engl J Med 2014. Randomised trial, 264 people, 60 weeks. PubMed 24836309
13. Sinaeinejad et al. Addict Health 2025. Randomised trial, 69 people, 8 weeks; no funding or conflicts declared. PubMed 41431629
14. Peng et al. Gen Hosp Psychiatry 2024. Pooled analysis of 12 trials, 904 people. PubMed 39504621
15. Sarris et al. CNS Drugs 2015. Randomised trial, 44 people, 16 weeks. PubMed 26374743
16. Costa et al. J Clin Psychiatry 2017. Randomised trial, 40 people, 16 weeks. PubMed 28617566
17. Grant et al. Arch Gen Psychiatry 2009. Randomised trial, 50 people, 12 weeks; funding not checked. PubMed 19581567
18. Grant et al. JAMA Psychiatry 2016. Randomised trial, 66 people, 12 weeks; funding not checked. PubMed 27007062
19. Thakker et al. Obstet Gynecol Int 2015. Pooled analysis of 8 trials, 910 women; trial doses from the full text. PubMed 25653680
20. Zhou et al. Andrologia 2021. Pooled analysis of 3 trials, 431 men. PubMed 33405232
21. Paschalis et al. Free Radic Biol Med 2018. Placebo-controlled crossover trial, 36 people, 30 days; funding not checked. PubMed 29233792
22. Coppersmith et al. Sci Rep 2021. Randomised crossover trial, 49 people. PubMed 34183702
23. Podobnik et al. Toxics 2024. Randomised trial, 40 people. PubMed 39195687
24. Sekhar et al. Am J Clin Nutr 2011. Uncontrolled study, 16 people, 2 weeks. PubMed 21795440
25. Kumar et al. J Gerontol A 2023. Randomised trial, 24 older adults, 16 weeks; government and charity funded, no conflicts declared. PubMed 35975308
26. Lizzo et al. Front Aging 2022. Randomised trial, 114 people, 2 weeks; most authors employed by a nutrition company. PubMed 35821844
27. Calverley et al. Drug Saf 2021. Review of 41 studies; sponsored by a NAC medicine maker. PubMed 33326056
28. Retraction notice, Reprod Biomed Online 2025, for a 2010 trial of 60 women. PubMed 41206153
29. Shahveghar Asl et al. Br J Nutr 2023. Pooled analysis of 18 studies, 2,185 women; cited for its retractions only. PubMed 36597797
30. Montes et al. Skinmed 2012. Randomised trial of a topical gel, 99 people. PubMed 23346662

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