N-Acetyl Cysteine
A form of the amino acid cysteine, taken most often for chesty mucus.
- Chestiness and mucus
- Antioxidants and inflammation
- Ovulation and hormones
- Sperm quality
- Exercise and training
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is N-Acetyl Cysteine?
N-acetyl cysteine, usually shortened to NAC, is the amino acid cysteine with an acetyl group attached; it turns up naturally in onions. Cells use cysteine to build glutathione, the antioxidant they lean on most, and reviews describe NAC as the precursor that hands them the raw material. It has been used as a medicine since the 1960s: the World Health Organization lists it as an antidote in poisoning, and it has long been given as a mucolytic, the sort of medicine that thins sticky mucus. The same compound is sold as a food supplement, and reviews call it cheap and widely available.
Why do people take N-Acetyl Cysteine?
Chestiness and mucus
The thinking. Sticky, stubborn mucus is the oldest reason people take NAC, and the one it has been used for longest as a medicine: reviews describe it as a mucolytic, a substance that thins mucus so it shifts more easily. A second idea sits behind that one, which is that an antioxidant might damp the oxidative strain in inflamed airways.
What research has found. Reviews pooling randomised trials in adults with a long-term lung condition measured flare-ups, lung function and questionnaire scores at 1,200 mg a day, and found no clear difference from a dummy capsule. A pooled comparison across 2,753 adults found NAC shortened flare-ups when they came, though it did not change how often they arrived or how often people went into hospital. A fair body of trials has looked at this, and most point the same way.
Antioxidants and inflammation
The thinking. Plenty of people take NAC for no single symptom, but as an antioxidant. It is the supplement aisle’s usual route to glutathione, which cells use to mop up the reactive molecules thrown off when they burn fuel, and reviews say NAC scavenges some of those molecules directly too. Glutathione levels drop as people age, and much of the interest follows from that.
What research has found. Trials here measure markers in the blood rather than how anyone feels, and the two reviews that pooled them do not agree on which markers moved. One found C-reactive protein and interleukin-6 lower than on a dummy capsule and nothing else shifting; the other found those two unchanged and three different markers lower. Doses ran from 400 to 2,000 mg a day, over one week to eighty. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
Ovulation and hormones
The thinking. Among women with polycystic ovary syndrome, NAC is one of the better-known supplements: reviews set it beside the medicine usually prescribed, and it comes up wherever irregular cycles and ovulation are discussed. The thinking runs through oxidative strain and insulin, with reviews describing NAC as damping the first and as acting on the signals insulin uses inside cells.
What research has found. Pooled analyses measured hormones and ovulation in women with the syndrome. The largest, covering 2,185 women, found total testosterone a little lower and follicle-stimulating hormone higher than in the comparison groups, while follicle counts, womb-lining thickness, progesterone, luteinising hormone and sex hormone-binding globulin were no different. Other reviews cover the same ground without settling on an overall figure. The studies disagree, so this is unsettled: dose, duration and who took part all seem to shift the result.
Sperm quality
The thinking. Men trying for a baby are a steady part of who buys NAC, and reviews list male fertility among the things it has been studied for. The argument is the antioxidant one: semen in men with fertility problems carries more of the reactive molecules that damage sperm and their DNA, and NAC is a familiar way to add an antioxidant.
What research has found. Two reviews pooling randomised trials in men with fertility problems measured semen samples after about three months. Both found more sperm, more of them moving and more of them normally shaped than in the comparison groups, one also found a larger ejaculate, and neither found side effects reported in the trials it gathered. A single trial of 50 men at 600 mg a day pointed the same way. Research here is still young: a few studies so far, mostly pointing the same way.
Exercise and training
The thinking. NAC has a niche among people who train hard, where it is sold as a sports antioxidant. The idea is that heavy exercise throws off reactive molecules faster than the body clears them, and that keeping glutathione topped up through a session keeps that in check.
What research has found. The two reviews here disagree. One pooled seven trials into a single performance figure and found an average change of about a third of one per cent, too small to tell apart from chance. The other read sixteen controlled trials in adult men and reported better performance in most of them, with no clear pattern for blood counts, inflammation or muscle measures. Doses in this field are unusually large: typically 5.8 g a day, and up to 20 g. Results here go both ways, which usually means the dose, the length of a trial or the group taking part makes the difference.
What does N-Acetyl Cysteine do in the body?
Swallowed, NAC is broken down to cysteine. Cysteine is usually the piece cells run short of when they make glutathione, the antioxidant they use most, so reviews describe NAC as the precursor that gets glutathione production going. It also scavenges some reactive oxygen molecules directly, without waiting to be converted.
Two further properties explain the rest of its reputation. Reviews describe it as anti-inflammatory as well as antioxidant, and as a mucolytic, a substance that makes thick mucus thinner and easier to shift, which is what put it in medicine cabinets in the first place. Glutathione falls as people age, and reviews point to that as one reason interest in NAC has grown. Very high doses, given in hospital by mouth or by drip, are the long-standing antidote for paracetamol overdose; the amounts on a supplement label are a small fraction of those.
What do people say about taking N-Acetyl Cysteine?
These are anecdotes people have shared, not evidence.
What people like
- Very commonly in the chest threads, mucus that comes up more easily: posters describe it as looser rather than less, so coughing clears more.
- Often, people say they simply tolerate it: weeks or months in, with nothing to report, which in supplement threads counts as a quiet success.
- Some describe a lift in energy in the first weeks, and one or two easier breathing during hard exercise within about a week.
- A few in the fertility threads report cycles changing within a month, though all of them were taking several other supplements at the same time.
Common complaints
- Most commonly, nothing happens: posters who gave it four to eight weeks for mucus or for energy report no change they could point to.
- Often, the early effect fades: an energy lift that lasted about a fortnight, after which some cycle on and off to chase it back.
- Some report side effects, chiefly heart palpitations, reflux or a stomach that disagreed with it, and in one case low mood that lifted on stopping.
- A few in the chest threads got the opposite of what they wanted: more mucus overall, or a dry cough that started with the capsules.
How people take it
- 600 mg twice a day is the dose named most often, with 600 mg once daily common and a few taking rather more.
- Splitting the day’s amount rather than swallowing it in one go is widely advised, and most posters use capsules, a few a powder.
- Timing splits the threads: some take it on an empty stomach, others with food and a large glass of water to settle the stomach.
- Almost nobody takes it alone: in the fertility threads it sits alongside inositol, folate and omega-3, and elsewhere alongside magnesium.
- Most give it a month or two before judging it, and several chest posters say a doctor suggested it or raised the dose.
Read from these public discussions:
- Has anyone tried N-A-C or Quercetin (two different OTC supplement) to reduce mucus production? (BronchAndNTM360social, 2019)
- NAC (supplement) - anyone tried this? Have a question! (Mumsnet, 2023)
- PCOS and TTC (Mumsnet, 2021)
- Anybody taken NAC alongside Myo-inositol and/or Metformin (Mumsnet, 2021)
- NAC supplement? (Mountain Bike Review forum, 2007)
What forms of N-Acetyl Cysteine are there?
| Form | What’s different | Per serving, on products we list |
|---|---|---|
| NAC | The amino acid cysteine with an acetyl group attached, the form used in supplements and medicines. | 700 mg |
What else is worth knowing about N-Acetyl Cysteine?
- Labels give the amount per serving, which may be more than one capsule. Trials in adults with a lung condition used 1,200 mg a day, and the fertility trial 600 mg.
- Side effects came up rarely in the trials. One review of sports research found no clear difference in side effects between NAC and a dummy capsule, though it noted reports rising as the dose rose, in a field where doses reached 20 g a day.
- Most of this research was done in people already under medical care: adults with a long-term lung condition, women with polycystic ovary syndrome, men with fertility problems. There is less to go on for healthy adults taking it day to day.
- NAC is a medicine as well as a supplement. Reviews describe it as the long-standing hospital antidote for paracetamol overdose, given by mouth or by drip at doses far above anything on a supplement label.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (19 studies)
- The efficacy of N -acetylcysteine in chronic obstructive pulmonary disease patients: a meta-analysis. (2023, meta-analysis)
- Efficacy and safety profile of mucolytic/antioxidant agents in chronic obstructive pulmonary disease: a comparative analysis across erdosteine, carbocysteine, and N-acetylcysteine. (2019, other)
- The effects of N-acetylcysteine on inflammatory and oxidative stress biomarkers: A systematic review and meta-analysis of controlled clinical trials. (2020, meta-analysis)
- The effects of N-Acetylcysteine on serum level of inflammatory biomarkers in adults. Findings from a systematic review and meta-analysis of randomized clinical trials. (2020, meta-analysis)
- Effects of N-acetylcysteine, oral glutathione (GSH) and a novel sublingual form of GSH on oxidative stress markers: A comparative crossover study. (2015, randomised trial)
- A review of antioxidant N-acetylcysteine in addressing polycystic ovary syndrome. (2024, systematic review)
- The effects of N-acetylcysteine on ovulation and sex hormones profile in women with polycystic ovary syndrome: a systematic review and meta-analysis. (2023, meta-analysis)
- The effects of N-acetylcysteine supplement on metabolic parameters in women with polycystic ovary syndrome: a systematic review and meta-analysis. (2023, systematic review)
- Unlocking the potential of antioxidant supplementation with n-acetylcysteine to improve seminal parameters and analysis of its safety: a systematic review and meta-analysis of randomized controlled trials. (2025, meta-analysis)
- Meta-analysis of the efficacy and safety of L-carnitine and N-acetylcysteine monotherapy for male idiopathic infertility. (2025, meta-analysis)
- Effects of N-acetyl-cysteine supplementation on sperm quality, chromatin integrity and level of oxidative stress in infertile men. (2019, randomised trial)
- Influence of N-Acetylcysteine Supplementation on Physical Performance and Laboratory Biomarkers in Adult Males: A Systematic Review of Controlled Trials. (2023, systematic review)
- Performance and Side Effects of Supplementation with N-Acetylcysteine: A Systematic Review and Meta-Analysis. (2017, meta-analysis)
- Overview on the Effects of N -Acetylcysteine in Neurodegenerative Diseases. (2018, review)
- A Review on Various Uses of N-Acetyl Cysteine. (2017, review)
- N-Acetylcysteine: A Review of Clinical Usefulness (an Old Drug with New Tricks). (2021, review)
- Rationale for the use of N-acetylcysteine in both prevention and adjuvant therapy of COVID-19. (2020, review)
- Comparing N-acetylcysteine and 4-methylpyrazole as antidotes for acetaminophen overdose. (2022, review)
- Medical and Dietary Uses of N-Acetylcysteine. (2019, review)
How much does N-Acetyl Cysteine cost?
That depends on the form and the pack, so we work out the price of the same dose of every N-Acetyl Cysteine product we can find and rank them.