N-Acetylcysteine (NAC)
Also known as: NAC, N-Acetyl-L-cysteine, Acetylcysteine, Mucomyst, Acetadote, Fluimucil
Mucolytic | Glutathione Precursor | FDA-Approved Drug
Overview
N-acetylcysteine (NAC) is the acetylated form of the amino acid L-cysteine and the rate-limiting precursor to glutathione, the body's principal endogenous antioxidant. NAC has been a prescription drug for several decades — most notably as a mucolytic (Mucomyst) for cystic fibrosis and COPD, and as an IV antidote (Acetadote) for acetaminophen overdose, where it remains the standard of care. It is also widely used as an over-the-counter supplement for liver support, antioxidant defence, and a growing list of investigational psychiatric applications including OCD, trichotillomania, and substance-use disorders. The supplement market for NAC has been turbulent — the FDA briefly contested its OTC status in 2020-2022 — but it is currently sold legally as a dietary supplement in the US and as a regulated drug in much of Europe and the UK. Mechanistically, NAC works as both a direct free-radical scavenger and as a substrate for glutathione synthesis; it also has independent effects on glutamatergic neurotransmission via the cystine-glutamate antiporter, which is the basis for its psychiatric applications.
Key benefits
- Replenishes hepatic glutathione, supporting liver detoxification capacity
- FDA-approved standard of care for acetaminophen overdose (IV form)
- Mucolytic effects useful in COPD, cystic fibrosis, and acute respiratory illness
- Reduces oxidative stress markers in clinical trials (MDA, F2-isoprostanes)
- Emerging evidence for OCD, trichotillomania, and skin-picking disorders
- May reduce craving and relapse in alcohol- and cocaine-use disorders
- Modest cardioprotective effects via reduction of homocysteine and oxidative LDL
- Generally well-tolerated with a wide therapeutic window
Mechanism of action
NAC's primary mechanism is supplying cysteine — the rate-limiting amino acid for glutathione (GSH) synthesis. Once absorbed, NAC is rapidly deacetylated to cysteine in the gut wall and liver, raising intracellular cysteine pools and driving GSH synthesis via γ-glutamylcysteine synthetase. Glutathione is the principal endogenous antioxidant in nearly every tissue, particularly important in hepatocytes (acetaminophen detoxification), red blood cells, and neurons. Beyond the GSH pathway, NAC has a free thiol group that directly reduces reactive oxygen species. In acetaminophen overdose, NAC replenishes GSH stores depleted by the toxic metabolite NAPQI, preventing hepatocyte necrosis. As a mucolytic, NAC's free thiol cleaves disulfide bonds in mucin glycoproteins, reducing mucus viscosity. In the CNS, NAC modulates the cystine-glutamate antiporter (system Xc-), which exchanges extracellular cystine for intracellular glutamate. The resulting elevation of extracellular glutamate appears to dampen pathological glutamatergic signalling in conditions like OCD and addiction — the basis for NAC's growing psychiatric evidence base.
Molecular data
- Type
- Acetylated cysteine derivative (C5H9NO3S)
- Half-life
- ~6 hours (oral); plasma cysteine elevation persists ~12 hours
Indications
What the research community uses this compound for, with self-reported effectiveness.
Liver & Detoxification
Acetaminophen Overdose
Most effectiveFDA-approved indication. IV NAC initiated within 8 hours of acetaminophen overdose prevents hepatocyte necrosis with near-100% efficacy by replenishing glutathione. Standard hospital protocol.
General Liver Support
ModerateDaily oral NAC raises hepatic GSH and reduces markers of oxidative liver stress. Useful in NAFLD, alcohol-related liver injury, and as a hepatoprotective adjunct on cycles using 17-α-alkylated orals or other hepatically-stressful compounds.
Respiratory
COPD Exacerbation Reduction
ModerateCochrane review of high-dose NAC (1200-1800 mg/day) shows modest reductions in COPD exacerbation rates and improvements in lung function. Effect size is small but consistent across multiple trials.
Acute Respiratory Illness
ModerateShort-course NAC reduces mucus viscosity and may shorten the duration of bronchitic symptoms in acute illness. Most clinical evidence is in chronic respiratory disease; the acute-illness use is largely empirical.
Mental Health
Obsessive-Compulsive Disorder (Adjunct)
ModerateMultiple RCTs show NAC at 2400-3000 mg/day as an adjunct to SSRIs reduces Y-BOCS scores by 4-7 points more than placebo over 12-16 weeks. Particularly promising for treatment-resistant cases.
Trichotillomania & Skin-Picking
EffectiveGrant 2009 RCT and follow-up studies show NAC 1200-2400 mg/day reduces hair-pulling and skin-picking behaviours significantly versus placebo. One of the few interventions with positive evidence in these conditions.
Substance-Use Disorders (Adjunct)
Under researchEmerging evidence for reduced craving and relapse in cocaine, cannabis, and nicotine use disorders. Mechanism is glutamatergic — restoring tonic glutamate signalling in addiction-relevant circuits.
Antioxidant / Cellular
Oxidative Stress Reduction
EffectiveReliably reduces serum and tissue markers of oxidative damage (MDA, 8-OHdG, F2-isoprostanes) across diverse populations. Clinical translation to hard outcomes is variable.
Dosing protocols
Common protocols by delivery method. Adjust the curve below to model accumulation in your own cycle.
Oral
Bioavailability: ~10% systemic (low — most is taken up by gut and liver, where its main effect is required)Sulfur smell is normal. Effervescent forms are best tolerated; capsules with food work for sensitive stomachs.
| Protocol | Dose | Frequency | Route / duration |
|---|---|---|---|
| General antioxidant / liver support Lowest evidence-supported daily dose for general use. | 600 mg | 1-2x daily | Ongoing |
| OCD or trichotillomania adjunct Higher dose used in psychiatric RCTs. Titrate up to minimise GI side effects. | 2400-3000 mg | Split into 2-3 doses daily | Minimum 12 weeks for evaluation |
| COPD adjunct Cochrane meta-analysis dose range. | 1200-1800 mg | 2-3x daily | Ongoing |
Intravenous
Bioavailability: 100% (hospital use only)IV protocol for acetaminophen overdose — administered in emergency departments, not relevant to outpatient supplementation.
Interactions
Pairwise interactions with other compounds you might be cycling — the same data behind the cycle builder's safety check.
Nitroglycerin
MonitorNAC potentiates nitroglycerin's vasodilatory effect — risk of severe hypotension. Significant only in cardiology contexts.
Activated charcoal
MonitorCharcoal binds NAC if co-administered. In overdose protocols, separate by 1-2 hours.
Glutathione (oral)
CompatibleOften used together in liver-support stacks — NAC supplies the building block; oral GSH supplies the finished molecule (though oral GSH bioavailability is debated).
Selenium / Vitamin C
SynergisticAll three contribute to the glutathione redox cycle. Common antioxidant stack.
Safety
Reported adverse effects, contraindications, and what to monitor on cycle.
Common side effects
- Sulfur taste/smell (the compound itself)
- Mild GI upset (nausea, loose stools), especially at >1200 mg single dose
- Headache (uncommon at OTC doses)
Uncommon side effects
- Diarrhoea or vomiting at high doses
- Anaphylactoid reaction with IV administration (not oral)
Rare side effects
- Bronchospasm with inhaled NAC in asthmatic patients
- Skin rash
Contraindications
- Active asthma exacerbation (relative contraindication for inhaled NAC; oral is fine)
Monitoring
- None required for typical supplemental doses. High-dose psychiatric protocols — check liver enzymes annually as a baseline practice.
References
Primary literature and clinical-trial data informing this entry.
Cazzola M et al. Influence of N-acetylcysteine on chronic bronchitis or COPD exacerbations: a meta-analysis. Eur Respir Rev. 2015;24(137):451-61.
COPD meta-analysis.
Grant JE et al. N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study. Arch Gen Psychiatry. 2009;66(7):756-63.
Foundational trichotillomania RCT.
Costa M et al. N-acetylcysteine treatment for substance use disorders: a systematic review. Brain Behav. 2021;11(1):e01943.
Review of NAC in addiction.
Last updated . How this page was sourced is described in the methodology.
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