Benefits
Antioxidant Support
NAC supplies cysteine, a building block of the antioxidant glutathione. In a 3-month trial in 120 men with unexplained infertility, 600 mg a day raised total antioxidant capacity and lowered oxidative stress markers versus placebo, and in a transplant trial oral NAC lowered reactive oxygen species in bone marrow cells. Effects in healthy people were not measured.
Respiratory Health and airway mucus clearance
NAC is classed as a mucolytic, a medicine believed to make mucus thinner and easier to cough up. In people with COPD, 600 mg twice daily for a year reduced flare-ups in a 1,006-person manufacturer-funded trial and a 120-person trial, but 600 mg once daily for 3 years had no effect in a 523-person trial. A Cochrane review of 38 mucolytic trials found a small reduction in flare-ups.
Mood and mental well-being support
In 75 adults with bipolar disorder on usual medication, 2 g a day of NAC for 24 weeks improved depression ratings versus placebo, but did not delay the next mood episode, and the gains faded after stopping. In 44 adults with OCD, 3 g a day for 16 weeks did not improve the main symptom score. NAC is not a substitute for psychiatric care.
Semen quality and ovulation support
In 120 men with unexplained infertility, 600 mg a day for 3 months improved semen volume, motility and viscosity versus placebo, but not sperm count or shape. In women with PCOS, a meta-analysis of eight trials (910 women) found higher odds of ovulation and pregnancy than placebo but lower than metformin, with no change in insulin, testosterone or BMI. Several of those trials had a high risk of bias.
Mechanism of action
Glutathione Precursor
NAC provides cysteine, a building block the body uses to make glutathione, an antioxidant produced inside cells.
Mucolytic Action
Mucolytics are believed to reduce the thickness of mucus so it is easier to cough up. NAC is one of the mucolytics pooled in the Cochrane review of people with chronic bronchitis or COPD.
Glutamate Modulation
NAC is thought to change glutamate release in parts of the brain by acting on the cystine-glutamate antiporter. This is the proposed rationale for the psychiatric trials, not a demonstrated effect in people who are well.
Acetaminophen antidote (hospital use)
In an analysis of 2,540 acetaminophen overdoses, oral NAC at 140 mg per kg, then 70 mg per kg every 4 hours for 17 doses, protected against liver damage when started within 8 hours. For a 70 kg adult the first dose alone is close to 10 grams. This is emergency drug treatment, not a reason to take NAC with acetaminophen at home.
Anti-Inflammatory Effects
Reviews describe anti-inflammatory activity alongside NAC's antioxidant effects.
Clinical trials
Double-blind randomized placebo-controlled trial of NAC 3 g/day (1.5 g twice daily) for 16 weeks; primary outcome the Yale-Brown Obsessive Compulsive Scale (YBOCS). (Sarris et al. 2015, CNS Drugs)
44 adults aged 18 to 70 with DSM-5 diagnosed OCD.
Null primary outcome: YBOCS total score did not differ from placebo (p = 0.39; per-protocol p = 0.15). The compulsions subscale favored NAC at week 12 but not week 16. Responders: 20% on NAC vs 27% on placebo. Heartburn was more common with NAC.
Open-label phase 3 trial: patients aged 15 to 60 with acute leukemia at high risk by a bone marrow endothelial cell marker were randomized 2 to 1 to oral NAC prophylaxis or none; 105 lower-risk patients were followed for comparison. Primary outcome: poor graft function or prolonged low platelets at day 60. (Wang et al. 2022, BMC Med)
120 randomized high-risk transplant patients, plus 105 lower-risk patients.
Poor graft function or prolonged low platelets occurred in 7.5% with NAC vs 22.5% without (hazard ratio 0.317, P = 0.021). Reactive oxygen species in marrow cells fell with NAC. Serious infections were similar (24% vs 25%). Open-label hospital supportive care, not general supplement use.
Double-blind placebo-controlled trial of NAC 600 mg twice daily for 1 year at one Hong Kong hospital. Outcomes: lung function, breathlessness, quality of life, 6-minute walk, flare-ups. (Tse et al. 2013, Chest)
120 adults aged 50 to 80 with stable COPD.
Small airway flow (P = .037) and forced oscillation measures improved, and flare-ups fell (0.96 vs 1.71 per year, P = .019). Breathlessness, quality of life and walking distance did not differ from placebo. One author worked for Zambon, an NAC maker. No major adverse effects.
Randomized double-blind placebo-controlled trial at 34 hospitals in China; NAC 600 mg twice daily for 1 year; primary outcome annual flare-up rate. Funded by Hainan Zambon Pharmaceutical. (Zheng et al. 2014, Lancet Respir Med)
1,006 adults aged 40 to 80 with moderate-to-severe COPD.
Flare-ups were 1.16 vs 1.49 per patient-year (risk ratio 0.78, p = 0.0011). Adverse events were similar: 29% with NAC vs 26% with placebo.
Randomized placebo-controlled trial at 50 centers; NAC 600 mg once daily for 3 years; primary outcomes yearly FEV1 decline and flare-ups per year. (Decramer et al. 2005, Lancet)
523 adults with COPD.
Null: FEV1 decline (54 vs 47 mL per year) and flare-ups (1.25 vs 1.29 per year, p = 0.85) did not differ from placebo. A subgroup analysis suggested fewer flare-ups in people not using inhaled corticosteroids. The authors concluded NAC was ineffective at this dose.