NAC (N-acetylcysteine)
NAC is a licensed medicine in the UK and US, used to thin mucus and, in hospital, as the antidote to paracetamol overdose; as a supplement, its best evidence is moderate, for modestly cutting flare-ups of COPD (chronic obstructive lung disease) at 600 mg twice a day. It is generally well tolerated, but it can trigger bronchospasm, especially in people with asthma, and may add to the blood-pressure-lowering effect of nitroglycerin.
Its use for flu-like illness, hair pulling, low mood and PCOS rests on small or mixed trials.
At a glance
Last reviewed 10 October 2026What it's used for
Evidence is rated per use. The same ingredient can be strong for one thing and traditional for another.
| Use | Evidence | What the research shows |
|---|---|---|
| Preventing COPD and chronic bronchitis flare-ups | Moderate | A 2019 Cochrane review of 38 trials of mucolytics, NAC among them, found that treating 8 people for about 9 months kept one more free of flare-ups (moderate certainty), though newer trials showed less benefit than older ones. For NAC itself, the 3-year BRONCUS trial (523 people, 600 mg once a day) found no fewer flare-ups, while PANTHEON (1,006 Chinese patients, 600 mg twice a day for a year, maker-funded) cut them from 1.49 to 1.16 per person per year.1,2,3 |
| Flu-like illness (winter prevention)Compare remedies for coughs & colds → | Limited | In one 1997 Italian trial of 262 mostly older adults, 600 mg twice a day for 6 months led to fewer, milder flu-like episodes and less time in bed than placebo. NAC did not stop flu infection: similar numbers caught the virus, but only 25% of those infected on NAC had symptoms, against 79% on placebo. It has not been repeated, so we rate it 2.4 |
| Hair pulling (trichotillomania) | Limited | A 2021 Cochrane review found one adult trial (50 people) in which 56% responded to NAC over 12 weeks against 16% on placebo (moderate certainty), but in the one trial in children and teenagers (39 people) NAC did no better than placebo (25% against 21%). The review called the evidence preliminary.5 |
| Depressive symptoms (add-on to treatment)Compare remedies for low mood → | Limited | A 2024 meta-analysis of 12 trials (904 people with depression or bipolar disorder, mostly already on treatment) found NAC at 1,000–3,000 mg a day for 8–24 weeks lowered depression scores slightly more than placebo; the effect was small (standardised mean difference −0.24) and the authors say larger trials are needed. It was tested as an add-on, not a replacement for treatment.6 |
| PCOS: ovulation and pregnancyCompare remedies for women’s health → | Limited | A 2015 meta-analysis of 8 trials (910 women with PCOS) found NAC at 1,200–1,800 mg a day raised ovulation and pregnancy rates compared with placebo, but did worse than metformin on both. The live-birth result came from a single 60-woman trial, and the authors flagged poor reporting and high risk of bias in several studies.7 |
How much
Ranges below are what studies used and what supplements commonly contain. They are not a recommendation for you. Start at the low end.
How people take it
Safety
Who should avoid
- Asthma — bronchospasm can occur; stop at once if breathing tightens (UK label)
- Past peptic ulcer — the UK label advises caution
- Children under 2 — mucolytics can block small airways
- Taking a cough suppressant — the UK label says not to combine them
- Suspected paracetamol overdose — get emergency care, don't self-treat with NAC
- Pregnancy and breastfeeding — limited data; ask a doctor or pharmacist first
Known interactions
Questions people ask
Is NAC legal as a supplement in the US?
It is sold, but its status is unsettled. The FDA says NAC was approved as a drug in 1963 before it was sold as a supplement, so it is excluded from the legal definition of a dietary supplement; in August 2022 it said it would not take action against NAC supplements that make no disease claims, while it considers a rule to allow them.
As of October 2026 we found no such rule published.
Can I use NAC for a paracetamol overdose?
No. Hospitals give acetylcysteine as the antidote, with the dose and need for treatment judged from blood tests and the time since the overdose. If someone may have taken too much paracetamol, call emergency services, even if they feel well.
Why does NAC smell of sulphur?
The smell comes from the molecule itself, which contains sulphur. The UK label says a mild sulphur odour is normal and does not mean the product has gone off.
What are the side effects of NAC?
The UK label lists stomach upset, nausea, vomiting, diarrhoea, headache and allergic reactions, and says bronchospasm can occur, especially in asthma. It also notes that studies found NAC reduces how platelets clump, though the clinical significance is not known, and bleeding (haemorrhage) is listed among its possible side effects.
In the year-long COPD trial, side effects were about as common on NAC (29%) as on placebo (26%).
Sources (10)
- Cochrane systematic review Poole P, Sathananthan K, Fortescue R. Mucolytic agents versus placebo for chronic bronchitis or chronic obstructive pulmonary disease. Cochrane Database Syst Rev. 2019.
- Randomised controlled trial Zheng JP, Wen FQ, Bai CX, et al. Twice daily N-acetylcysteine 600 mg for exacerbations of chronic obstructive pulmonary disease (PANTHEON): a randomised, double-blind placebo-controlled trial. Lancet Respir Med. 2014.
- Randomised controlled trial Decramer M, Rutten-van Mölken M, Dekhuijzen PN, et al. Effects of N-acetylcysteine on outcomes in chronic obstructive pulmonary disease (Bronchitis Randomized on NAC Cost-Utility Study, BRONCUS): a randomised placebo-controlled trial. Lancet. 2005.
- Randomised controlled trial De Flora S, Grassi C, Carati L. Attenuation of influenza-like symptomatology and improvement of cell-mediated immunity with long-term N-acetylcysteine treatment. Eur Respir J. 1997.
- Cochrane systematic review Hoffman J, Williams T, Rothbart R, Ipser JC, Fineberg N, Chamberlain SR, Stein DJ. Pharmacotherapy for trichotillomania. Cochrane Database Syst Rev. 2021.
- Systematic review and meta-analysis Peng TR, Lin HH, Tseng TL, et al. Efficacy of N-acetylcysteine for patients with depression: An updated systematic review and meta-analysis. Gen Hosp Psychiatry. 2024.
- Systematic review and meta-analysis Thakker D, Raval A, Patel I, Walia R. N-acetylcysteine for polycystic ovary syndrome: a systematic review and meta-analysis of randomized controlled clinical trials. Obstet Gynecol Int. 2015.
- Regulator guidance US Food and Drug Administration. Policy Regarding N-acetyl-L-cysteine: Guidance for Industry. August 2022.
- Regulator label electronic medicines compendium (emc). NACSYS 600 mg Effervescent Tablets: Summary of Product Characteristics, Alturix Limited. Revised July 2023.
- Regulator label DailyMed. Acetylcysteine solution (for inhalation or oral use), Hospira, Inc.: prescription drug label. Updated May 2026.
Last reviewed 10 October 2026 · Next review April 2027 · How we rate evidence
Reader notes
Experiences and questions from readers, read by an editor before they appear. They are personal accounts, not evidence, and are not medical advice.