Evidence-based summary of human clinical trials for N-acetyl cysteine (NAC), covering glutathione support, respiratory health, psychiatric conditions, liver-related clinical use, and fertility-related outcomes.

This page summarizes peer-reviewed human clinical trials investigating the effects of NAC across several medical and supplemental-use contexts.

Quick Answer

NAC is a widely studied compound that serves as a precursor to glutathione and has also been used in respiratory, liver-related, and psychiatric research. The strongest evidence supports its established medical role in acetaminophen overdose treatment and its use in some chronic respiratory conditions, while adjunctive psychiatric and fertility-related applications remain promising but more variable across studies. NAC is best understood as a multi-context compound with clinically relevant evidence in some settings and emerging evidence in others.

The studies summarized below represent selected human clinical trials and reviews frequently cited in the literature.

Mechanism Overview

NAC (N-Acetyl-L-Cysteine) is an acetylated form of the amino acid L-cysteine. Its effects are commonly discussed through several mechanisms:

Glutathione precursor: NAC supplies cysteine, which is relevant to glutathione synthesis.
Direct antioxidant activity: The thiol group is often discussed in relation to redox balance and oxidative-stress pathways.
Mucolytic action: NAC can reduce mucus viscosity by disrupting disulfide bonds in mucus glycoproteins.
Liver-related clinical use: NAC is used clinically in acetaminophen overdose because of its role in restoring hepatic glutathione-related defenses.
Glutamate modulation: NAC has been investigated for effects on glutamate homeostasis, especially in psychiatric and addiction-related research.
Inflammatory signaling: Some studies discuss effects on inflammatory pathways including NF-κB and cytokine signaling.

Human Clinical Trials Summary

Respiratory Health (COPD, Bronchitis, Mucus)

Study	Design	Dose	Duration	Key Findings	PMID
Decramer et al., 2005 (BRONCUS)	RCT, n=523	600mg/day	3 years	No significant effect on FEV1 decline overall; reduced exacerbations in a subgroup not using inhaled corticosteroids.	15939842
Zheng et al., 2014 (PANTHEON)	RCT, n=1,006	600mg 2x/day	1 year	Reduced acute COPD exacerbations versus placebo in the study population.	24477426
Stey et al., 2000 (Meta-analysis)	11 RCTs, n=2,011	400–600mg/day	3–6 months	Meta-analysis supported reduced exacerbation frequency and symptom improvement versus placebo in chronic bronchitis contexts.	10905768
Hansen et al., 1994	RCT, n=153	600mg/day	6 months	Reported fewer symptom exacerbations and sick days in chronic bronchitis patients.	7989533

Psychiatric & Neurological Applications

Study	Design	Dose	Duration	Key Findings	PMID
Berk et al., 2008	RCT, n=75	1g 2x/day	6 months	NAC was studied as an adjunct in bipolar depression and was associated with improvement in several symptom and functioning measures.	18534556
Grant et al., 2009	RCT, n=27	1.2–2.4g/day	12 weeks	Trial results supported symptom reduction in trichotillomania versus placebo.	19564965
Afshar et al., 2012	RCT, n=48	1.2–2.4g/day	12 weeks	Adjunctive NAC was associated with reduction in OCD symptom scores in the trial population.	22867086
LaRowe et al., 2007	RCT, n=23	1.2–3.6g/day	4 weeks	Preliminary findings supported reduced craving and cocaine use in a small study.	17606664
Deepmala et al., 2015 (Systematic Review)	Review of multiple RCTs	600mg–3g/day	Variable	Systematic review summarized evidence across addiction, OCD, depression, bipolar disorder, and schizophrenia-related research.	25268486

Liver Protection & Detoxification

Study	Design	Dose	Duration	Key Findings	PMID
Prescott et al., 1979	Clinical series	IV protocol	Acute treatment	NAC is an established part of treatment protocols for acetaminophen overdose.	36695
Nguyen-Khac et al., 2011	RCT, n=174	IV NAC + prednisolone	1 month	In alcoholic hepatitis, adjunctive NAC was associated with lower short-term mortality in the study population.	21527579
Millea, 2009	Review	Variable	Variable	Review article discussing hepatoprotective mechanisms and clinical applications of NAC.	19111278

Fertility (Male & Female)

Study	Design	Dose	Duration	Key Findings	PMID
Safarinejad & Safarinejad, 2009	RCT, n=120	600mg/day	3 months	Reported improvements in semen-related parameters in infertile men.	19091332
Rizk et al., 2005	RCT, n=150	1.2g/day	5 days (cycle)	NAC plus clomiphene was associated with improved ovulation-related outcomes in women with PCOS.	15929848
Salehpour et al., 2012	RCT, n=100	1.2g/day + letrozole	5 days	Adjunctive NAC was associated with improved ovulation induction outcomes in the study setting.	22695252

Contrast-Induced Nephropathy Prevention

Study	Design	Dose	Duration	Key Findings	PMID
Tepel et al., 2000	RCT, n=83	600mg 2x/day	2 days (pre/post contrast)	Early trial results suggested reduced contrast-induced nephropathy incidence.	10894826
ACT Trial, 2011	RCT, n=2,308	1.2g IV + 1.2g oral 2x/day	4 doses	Large trial did not show significant benefit for prevention of contrast-induced acute kidney injury.	21920968

Dosages Used in Studies

Respiratory Health

Mucolytic use: 600mg 1–2x/day
COPD-related studies: 600mg 2x/day (1,200mg total)

Psychiatric Applications

Common study range: 1,000–2,400mg/day
Divided dosing is common
Some protocols begin at lower doses and titrate upward

Liver-Related Use

General supplemental-use contexts: 600–1,200mg/day
Acetaminophen overdose: IV protocol in clinical settings

Fertility

Male fertility studies: around 600mg/day
PCOS-related studies: around 1,200mg/day in adjunct protocols

Tolerability may differ depending on dose, timing, and clinical context.

Safety & Contraindications

NAC has a long clinical history and is generally well tolerated in many research settings, though side effects can occur.

Common side effects: Nausea, diarrhea, vomiting, and sulfur-like odor or taste
Less common: Headache, rash, or bronchospasm, especially with inhaled use in susceptible individuals
Drug interactions: Interaction discussions often involve nitroglycerin, activated charcoal in overdose settings, and certain testing contexts
Clinical context matters: Medical use, supplement use, and hospital-based protocols should not be treated as interchangeable

Note: NAC has both supplement-market history and established medical use. Product status and labeling can vary by jurisdiction and time period.

Evidence Summary

Acetaminophen Overdose: Definitive evidence (Grade A+) — NAC is an established part of standard medical treatment protocols.
COPD / Chronic Bronchitis: Strong evidence (Grade A−) — Multiple trials and reviews support benefit in some respiratory settings, especially exacerbation-related outcomes.
Psychiatric Disorders: Moderate evidence (Grade B) — Results are promising across several conditions, but outcomes remain variable and condition-specific.
Liver-Related Clinical Use: Moderate evidence (Grade B) — Strongest in overdose settings; broader hepatoprotective interpretation should remain cautious.
Fertility: Moderate evidence (Grade B) — Some positive trial results exist, but more replication is needed.
Contrast Nephropathy Prevention: Mixed evidence (Grade C) — Early studies were promising, while later large-trial results were less supportive.

References

Zheng JP, Wen FQ, Bai CX, et al. Twice daily N-acetylcysteine 600 mg for exacerbations of chronic obstructive pulmonary disease (PANTHEON). Lancet Respir Med. 2014;2(3):187-194. PMID: 24477426
Decramer M, Rutten-van Mölken M, Dekhuijzen PN, et al. Effects of N-acetylcysteine on outcomes in chronic obstructive pulmonary disease (BRONCUS). Lancet. 2005;365(9470):1552-1560. PMID: 15939842
Stey C, Steurer J, Bachmann S, et al. The effect of oral N-acetylcysteine in chronic bronchitis: a quantitative systematic review. Eur Respir J. 2000;16(2):253-262. PMID: 10905768
Berk M, Copolov DL, Dean O, et al. N-acetyl cysteine for depressive symptoms in bipolar disorder—a double-blind randomized placebo-controlled trial. Biol Psychiatry. 2008;64(6):468-475. PMID: 18534556
Grant JE, Odlaug BL, Kim SW. N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania. Arch Gen Psychiatry. 2009;66(7):756-763. PMID: 19564965
Afshar H, Roohafza H, Mohammad-Beigi H, et al. N-acetylcysteine add-on treatment in refractory obsessive-compulsive disorder. J Clin Psychopharmacol. 2012;32(6):797-803. PMID: 22867086
Deepmala, Slattery J, Kumar N, et al. Clinical trials of N-acetylcysteine in psychiatry and neurology: A systematic review. Neurosci Biobehav Rev. 2015;55:294-321. PMID: 25268486
Prescott LF, Illingworth RN, Critchley JA, et al. Intravenous N-acetylcysteine: the treatment of choice for paracetamol poisoning. Br Med J. 1979;2(6198):1097-1100. PMID: 36695
Safarinejad MR, Safarinejad S. Efficacy of selenium and/or N-acetyl-cysteine for improving semen parameters in infertile men. J Urol. 2009;181(2):741-751. PMID: 19091332
Tepel M, van der Giet M, Schwarzfeld C, et al. Prevention of radiographic-contrast-agent-induced reductions in renal function by acetylcysteine. N Engl J Med. 2000;343(3):180-184. PMID: 10894826

Last updated: March 2026. This page summarizes published peer-reviewed research and is not medical advice. Consult a healthcare provider before starting any supplement.

NAC (N-Acetyl Cysteine) Clinical Studies