NAC
Evidence Fact Sheet
N-Acetyl Cysteine
NAC (N-acetylcysteine) is a glutathione precursor and thiol antioxidant with a dual drug/supplement identity. Human meta-analyses show fewer COPD/chronic-bronchitis exacerbations and improved PCOS hormone markers; honest negatives exist for substance craving and routine contrast-nephropathy prevention.
Also known as: N-Acetylcysteine · N-Acetyl-L-cysteine · Acetylcysteine
Overview
NAC (N-acetyl-cysteine) is the acetylated form of the amino acid L-cysteine and the rate-limiting building block your body uses to make glutathione (GSH), its master intracellular antioxidant. Mechanistically it acts as a GSH precursor, a direct free-radical scavenger via its thiol (-SH) group, an NRF2 antioxidant-response activator, and a mucolytic that cleaves mucin disulfide bonds; it also modulates the cystine-glutamate antiporter (System Xc-), which underlies its glutamatergic research interest. It is studied across respiratory, liver, reproductive, metabolic, and neuropsychiatric contexts; consumer supplements are typically 600 mg/capsule with research doses of 600-1800 mg/day (psychiatry research up to 2400 mg/day under supervision). Regulatory status is unusual and market-specific: in the US, NAC is simultaneously an FDA-approved prescription drug (mucolytic / acetaminophen-overdose antidote) and a dietary supplement sold under structure/function claims with the mandatory DSHEA disclaimer (FDA announced enforcement discretion in 2022); it is a permitted antioxidant supplement constituent under ANVISA in Brazil, has no EFSA-authorized NAC-specific health claim in the EU, and in China holds drug identity only (not permitted as a supplement). This is an educational evidence summary, not medical advice or a dosing recommendation, and oral supplements are never a substitute for prescription NAC in any poisoning or overdose setting.
Mechanism of Action
Glutathione (GSH) precursor — deacetylated to L-cysteine, the rate-limiting substrate for GSH synthesis · Direct free-radical scavenging via the thiol (-SH) group · NRF2 antioxidant-response activation · NF-κB signaling suppression (indirect, via restored GSH) · Mitochondrial GSH pool restoration · Mucolysis — cleaves mucin disulfide bonds (research/pharmacological mechanism) · Glutamatergic modulation via cystine-glutamate antiporter (xCT / System Xc-)
Body systems: Immune System · Respiratory & Mucosal Barrier · Liver & Detoxification · Reproductive · Mitochondrial & Cellular Energy · CNS
Evidence-Based Benefits
Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — NAC is not characterized as a treatment for any disease.
Respiratory / COPD & Chronic-Bronchitis Exacerbations
Meta-analysis supported- RR 0.75exacerbations · 95% CI 0.66-0.84
- 13 studies4155 COPD patients
In the strongest pooled analysis to date, supplemental NAC was associated with significantly and consistently fewer chronic-bronchitis/COPD exacerbations versus placebo across 13 trials, with the authors noting that patients with spirometry-confirmed airway obstruction needed higher doses (≥1200 mg/day) while those without obstruction benefited at 600 mg/day. This is a clinical-population research finding, not a general-wellness claim.
Reported effect: Relative risk 0.75 (95% CI 0.66-0.84; p<0.01) for exacerbations across 13 studies / 4155 COPD patients (NAC n=1933 vs control n=2222); high-dose RR 0.75 (95% CI 0.68-0.82) in spirometry-diagnosed COPD
“patients treated with NAC had significantly and consistently fewer exacerbations of chronic bronchitis or COPD (relative risk 0.75, 95% CI 0.66-0.84; p < 0.01) ... 13 studies, 4155 COPD patients, NAC n = 1933; placebo or controls n = 2222 ... high doses of NAC were also effective in patients suffering from COPD diagnosed using spirometric criteria (relative risk 0.75, 95% CI 0.68-0.82; p = 0.04)”
Source: PMID 26324807 · Cazzola 2015 · European Respiratory Review
Respiratory / COPD vs Chronic-Bronchitis (Pre-COPD) Subgroups
Meta-analysis supported- IRR 0.76COPD exacerb · 95% CI 0.59-0.99
- IRR 0.81CB/pre-COPD · 95% CI 0.69-0.95
- 20 studiesdistinct meta-analyses
A 2024 set of distinct meta-analyses separated established COPD from earlier chronic-bronchitis/pre-COPD populations and found significant reductions in exacerbation incidence in both groups, plus a marked improvement in symptoms/quality of life in the chronic-bronchitis/pre-COPD subgroup. This refines the older pooled signal by showing benefit is not limited to advanced disease.
Reported effect: Exacerbation incidence rate ratio IRR=0.76 (95% CI 0.59-0.99) in COPD and IRR=0.81 (95% CI 0.69-0.95) in CB/pre-COPD; symptom/QoL improvement OR=3.47 (95% CI 1.92-6.26) in CB/pre-COPD; 20 studies (7 in CB/pre-COPD)
“NAC treated patients showed a significant reduction of the incidence of exacerbations as compared to placebo both in COPD (IRR=0.76; 95% confidence interval (CI) 0.59-0.99) and CB/pre-COPD (IRR=0.81; 95% CI 0.69-0.95) ... CB/pre-COPD patients treated with NAC were significantly more likely to experience an improvement in symptoms and/or QoL compared to placebo (odds ratio (OR)=3.47; 95% CI 1.92-6.26)”
Source: PMID 38555190 · Papi 2024 · Archivos de Bronconeumología
PCOS / Female Reproductive Hormone Markers
Meta-analysis supported- SMD −0.25testosterone · 95% CI −0.39,−0.10
- SMD 0.39FSH · 95% CI 0.07,0.71
- 18 studies2185 participants
Pooling 18 studies in women with polycystic ovary syndrome, NAC significantly lowered total testosterone and raised FSH. The same analysis is an honest mixed picture: it found no significant effect on follicle number, endometrial thickness, progesterone, LH, or sex-hormone-binding globulin. Framed as research on hormone markers in a studied clinical population.
Reported effect: Total testosterone SMD −0.25 ng/ml (95% CI −0.39 to −0.10; p<0.001); FSH SMD 0.39 mg/ml (95% CI 0.07 to 0.71; p=0.01); 18 studies, 2185 participants; no significant effect on follicle number, endometrial thickness, progesterone, LH, or SHBG
“Eighteen studies, including 2185 participants, were included in the present meta-analysis. ... NAC significantly reduced total testosterone (TT) levels (standardised mean difference (SMD): −0·25 ng/ml; 95 % CI (−0·39, −0·10); P < 0·001 ... increased follicle-stimulating hormone (FSH) levels (SMD: 0·39 mg/ml; 95 % CI (0·07, 0·71); P = 0·01 ... no significant effect was observed on the number of follicles, endometrial thickness, progesterone, serum luteinising hormone levels and sex hormone-binding globulin.”
Source: PMID 36597797 · Shahveghar Asl 2023 · British Journal of Nutrition
Male Reproductive / Sperm-Quality Markers
RCT supported- P<0.05count, motility, DNA frag
In an RCT of 50 infertile men with asthenoteratozoospermia, 600 mg/day NAC for 3 months was reported to significantly increase sperm count and motility and reduce abnormal morphology, DNA fragmentation, and the oxidative-stress marker MDA, with antioxidant capacity (TAC) rising. The abstract reports direction and significance (P<0.05) but no point estimates, so no effect size is extracted here.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“sperm count and motility increased significantly whereas abnormal morphology, DNA fragmentation and protamine deficiency showed significant decreases compared to pre-treatment levels (P < 0.05) ... TAC significantly increased and MDA decreased with an inverse significant correlation between TAC and MDA (P < 0.05)”
Source: PMID 30771790 · Jannatifar 2019 · Reproductive Biology and Endocrinology
Healthy Aging / Glutathione & Oxidative Stress (GlyNAC Combination)
RCT supported- 16 weeksRCT duration
- GlyNACglycine + NAC combo
A randomized trial in older adults reported that GlyNAC (glycine + NAC), not placebo, corrected age-related glutathione deficiency and oxidative stress and improved several aging-related measures over 16 weeks. Important framing note: the headline aging signal is the GlyNAC combination, not NAC monotherapy, and the abstract states directional correction without numeric point estimates, so no effect size is extracted.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“Twenty-four OA and 12 young adults (YA) were studied. OA was randomized to receive either GlyNAC (N = 12) or isonitrogenous alanine placebo (N = 12) for 16-weeks ... GlyNAC (and not placebo) supplementation in OA improved/corrected these defects”
Source: PMID 35975308 · Kumar 2023 · J Gerontol A Biol Sci Med Sci
Mood / Bipolar Depression (Adjunctive Research)
Meta-analysis supported- d = 0.45vs placebo · 95% CI 0.06-0.84
- 6 trials248 patients
A meta-analysis of NAC added on to standard treatment for bipolar depression found a moderate effect favoring NAC over placebo. The authors caution that the confidence interval is wide and call for larger trials, so the result is best read as a promising but not definitive research signal in a studied clinical population.
Reported effect: Cohen's d = 0.45 (95% CI 0.06-0.84) favoring NAC over placebo; 6 trials, 248 patients
“Six trials including 248 patients were included. Treatment augmentation with NAC showed a moderate effect size favoring NAC over placebo (d = 0.45, 95% C.I.: 0.06-0.84).”
Source: PMID 33354859 · Nery 2021 · Bipolar Disorders
Substance-Use Craving (Honest Negative)
Null / no benefit Meta-analysis supported- SMD 0.189NS · 95% CI −0.015 to 0.393
- 9 trials623 participants
- I² 99.26%very high heterogeneity
Despite mechanistic interest in NAC's glutamatergic action, a 2024 meta-analysis found NAC did not significantly outperform placebo for reducing substance-use cravings, with a confidence interval crossing zero and very high heterogeneity between trials. This is a key honest negative for the addiction research area.
Reported effect: SMD = 0.189 (95% CI −0.015 to 0.393), non-significant; 9 trials, 623 participants; heterogeneity I²=99.26%
“9 trials which analysed data from a total of 623 participants ... NAC did not significantly outperform placebo in reducing symptoms of craving in the meta-analysis (SMD = 0.189, 95% CI = -0.015-0.393) ... Heterogeneity was very high in the meta-analysis (99.26%)”
Source: PMID 39556483 · Winterlind 2024 · Addiction Biology
Schizophrenia Augmentation (Small / Nuanced Effect)
Null / no benefit Meta-analysis supported- 3 RCTsnegative symptoms ≥24 wk
- 2 RCTstotal psychopathology
- too smallto be clinically meaningful
A meta-analytic review of NAC added to antipsychotics found statistically significant attenuation of negative symptoms and total psychopathology only at 24 weeks or longer, but the author concluded the magnitude of benefit appeared too small to be clinically meaningful. Flagged as a mixed/limited finding rather than a clear positive. The abstract reports trial counts but no pooled effect size, so none is extracted.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“significant attenuation by NAC of negative symptom (3 RCTs) and total psychopathology (2 RCTs) ratings at ≥ 24 weeks ... reduction in psychopathology ratings, though statistically significant, appeared too small to be clinically meaningful.”
Source: PMID 36170201 · Andrade 2022 · J Clin Psychiatry
Contrast-Induced Nephropathy Prevention (Honest Negative)
Null / no benefit Meta-analysis supported- 34 studiesnot conclusively resolved
- 19 RCTs+4 nonrandomized +11 MAs
One of the most cited NAC research areas is also one of its clearest cautionary tales: this analysis showed that even after 19 RCTs and 11 prior meta-analyses, the accumulated evidence had not conclusively resolved whether acetylcysteine prevents contrast-induced nephropathy, a frequently cited example of inconsistent/heterogeneous trial data not supporting routine use. The abstract gives study counts but no pooled risk ratio, so none is extracted.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“34 empirical studies have not yet conclusively resolved this research question ... 19 randomized controlled trials, 4 prospective nonrandomized studies, and 11 meta-analyses”
Source: PMID 16432083 · Bagshaw 2006 · Archives of Internal Medicine
Immune / Pulmonary Tuberculosis Lung Function (Mixed)
Null / no benefit RCT supported- HR 0.84culture conversion · NS · p=0.33
- n=1401200 mg twice daily
- FVC +0.49lung recovery · 95% CI 0.02-0.95
In an RCT of 140 adults with moderate-to-advanced tuberculosis, adjunctive NAC at 1200 mg twice daily did NOT improve the primary endpoint of stable culture conversion (HR 0.84, NS). However, a prespecified secondary signal showed improved recovery of lung function (forced vital capacity). An honest mixed result: negative primary, supportive secondary.
Reported effect: Primary culture conversion hazard ratio 0.84 (95% CI 0.59-1.20; p=0.33), not significant; n=140; NAC 1200 mg twice daily; secondary FVC recovery NAC×month 0.49 (95% CI 0.02-0.95) and FEV1 0.42 (95% CI −0.06 to 0.91)
“enrolled 140 adults with moderate or far-advanced tuberculosis ... 1200 mg of oral NAC twice daily ... NAC...did not increase stable culture conversion (hazard ratio, 0.84; 95% CI, 0.59 to 1.20; P=0.33) ... NAC treatment was associated with improved recovery of lung function (NAC × month, 0.49 [95% CI, 0.02 to 0.95] and 0.42 [95% CI, -0.06 to 0.91] for forced vital capacity and forced expiratory volume in the first second)”
Source: PMID 39189858 · Wallis 2024 · NEJM Evidence
Dosage (research context · not a recommendation)
600-1800 mg/day (educational reference · most consumer products 600 mg/capsule · respiratory/mucus and reproductive RCTs at 600-1200 mg/day · psychiatry research up to 2400 mg/day under medical supervision · best absorbed away from food)
Regulatory Status · 4 Markets
- US · FDA
- Dual identity: FDA-approved prescription drug (acetylcysteine as a mucolytic and acetaminophen-overdose antidote, with roots in the 1960s-70s; the branded IV product Acetadote, NDA 021539, was approved in 2004) AND dietary supplement. In July 2020 FDA issued warning letters over illegal NAC hangover/disease claims, citing as the legal basis that NAC had been approved as a drug before DSHEA (1994) — though FDA notes its own pre-2016 NAC approval records are unreliable and conflicting; in 2022-08 FDA announced enforcement discretion, declining to act against dietary supplements solely for containing NAC. Supplement marketed under Structure/Function Claims + mandatory DSHEA disclaimer; permanent status still pending formal rulemaking.
- EU · EFSA
- No EU-wide harmonized supplement framework and NO EFSA-authorized NAC-specific health claim (Reg 432/2012). Approved as OTC mucolytic / Rx antidote in most member states (Fluimucil, ACC). Supplement status varies by country: Italy / Germany / France (often regulated as medicine). Cysteine carries a conditional Article 13.1 hair/nail/skin claim under specific conditions.
- CN · China
- China: NAC has drug identity only (mucolytic/antidote); not in health-food catalogue, not approved novel food, not a food additive, not. Selling as a supplement is non-compliant; use GSH or L-cysteine as alternative routes.
- BR · ANVISA
- Listed in ANVISA permitted constituents for dietary supplements (Suplemento Alimentar) under RDC 239/2018 + IN 28/2018; permitted use as antioxidant / sulfur-amino-acid supplement. Also marketed as a medicine (mucolytic / antidote). Functional claims restricted to ANVISA positive-list wording; requires Portuguese labeling and local regulatory representative.
Safety
Generally well tolerated orally at 600-1800 mg/day; GI upset (nausea, diarrhea, bloating) in ~10-15%, worse on empty stomach. Strong sulfur odor. Theoretical interactions: nitroglycerin (enhanced vasodilation/hypotension), activated charcoal (reduced NAC absorption), and additive caution with anticoagulants. Caution in active peptic ulcer; IV-route anaphylactoid reactions do not apply to oral use. No adequate human supplement data in pregnancy/lactation. Consult a healthcare provider. NOT a substitute for prescription NAC in any medical (poisoning/overdose) setting.
Related
Goals: longevity-stack · reproductive-health
Lifestyles: senior-60-plus
References
PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.
- PMID 26324807 · Cazzola 2015 · European Respiratory Review — Respiratory / COPD & Chronic-Bronchitis Exacerbations
- PMID 38555190 · Papi 2024 · Archivos de Bronconeumología — Respiratory / COPD vs Chronic-Bronchitis (Pre-COPD) Subgroups
- PMID 36597797 · Shahveghar Asl 2023 · British Journal of Nutrition — PCOS / Female Reproductive Hormone Markers
- PMID 30771790 · Jannatifar 2019 · Reproductive Biology and Endocrinology — Male Reproductive / Sperm-Quality Markers
- PMID 35975308 · Kumar 2023 · J Gerontol A Biol Sci Med Sci — Healthy Aging / Glutathione & Oxidative Stress (GlyNAC Combination)
- PMID 33354859 · Nery 2021 · Bipolar Disorders — Mood / Bipolar Depression (Adjunctive Research)
- PMID 39556483 · Winterlind 2024 · Addiction Biology — Substance-Use Craving (Honest Negative)
- PMID 36170201 · Andrade 2022 · J Clin Psychiatry — Schizophrenia Augmentation (Small / Nuanced Effect)
- PMID 16432083 · Bagshaw 2006 · Archives of Internal Medicine — Contrast-Induced Nephropathy Prevention (Honest Negative)
- PMID 39189858 · Wallis 2024 · NEJM Evidence — Immune / Pulmonary Tuberculosis Lung Function (Mixed)
Frequently Asked Questions
1. What is NAC and how is it thought to work?
NAC (N-acetyl-cysteine) is the acetylated form of the amino acid L-cysteine. Inside cells it is deacetylated to cysteine, the rate-limiting building block for synthesizing glutathione (GSH) — the body's master intracellular antioxidant. NAC also scavenges free radicals directly through its thiol (-SH) group, activates the NRF2 antioxidant-response pathway, and cleaves the disulfide bonds in mucus (its mucolytic mechanism). These are mechanistic descriptions from research, not promises of a health outcome.
2. What does the strongest human evidence actually show?
The most robust pooled evidence is in respiratory health: a meta-analysis of 13 trials in 4,155 COPD patients found significantly fewer chronic-bronchitis/COPD exacerbations on NAC (relative risk 0.75, 95% CI 0.66-0.84), and a 2024 set of meta-analyses confirmed reductions in both COPD (IRR 0.76) and earlier chronic-bronchitis/pre-COPD (IRR 0.81) populations. In PCOS, an 18-study, 2,185-participant meta-analysis showed NAC lowered total testosterone (SMD -0.25) and raised FSH (SMD 0.39). These are findings in specific studied populations, not general-wellness claims.
3. What are the honest negatives or disappointments for NAC?
Several. A 2024 meta-analysis of 9 trials (623 participants) found NAC did NOT significantly reduce substance-use cravings (SMD 0.189, 95% CI -0.015 to 0.393, non-significant, with very high heterogeneity). For contrast-induced nephropathy — once a heavily promoted use — even 19 RCTs and 11 prior meta-analyses had not conclusively shown benefit. In tuberculosis, a 140-person RCT missed its primary endpoint of culture conversion (HR 0.84, non-significant), though a secondary lung-function signal was positive. And in schizophrenia augmentation, a benefit reached statistical significance only at ≥24 weeks and was judged too small to be clinically meaningful.
4. What doses are used in research, and is the GlyNAC aging data about NAC alone?
Consumer supplements are typically 600 mg per capsule. Respiratory and reproductive trials generally use 600-1200 mg/day; psychiatry research has gone up to 2400 mg/day under medical supervision. Importantly, the headline 'healthy aging' data comes from GlyNAC — a combination of glycine plus NAC — not NAC monotherapy; the 16-week older-adult trial randomized 12 to GlyNAC versus 12 to placebo and reported correction of glutathione deficiency, so that signal should not be attributed to NAC alone. None of this is a dosing recommendation; doses and suitability should be discussed with a healthcare provider.
Last evidence review: 2026-06-13