NAC (N-acetylcysteine)

What NAC is and why it is a medicine

NAC is the acetylated derivative of the amino acid L-cysteine. The acetyl group makes it more stable and is split off in the body. Cysteine is one of the three building blocks of glutathione and the one that limits its production. That is the whole idea behind the substance.

This leads to a situation that surprises many: in Germany, NAC is not a food supplement but a medicine. The oral preparations are approved as expectorants, available only in pharmacies and without prescription; the antidote is a separate, prescription-only preparation. There is therefore no upper limit from EFSA or the BfR, but an approved dosage.

The benchmark: paracetamol poisoning

In the liver, paracetamol is converted into a highly reactive intermediate that is normally captured by glutathione. In an overdose, glutathione is used up and the liver cells die. Liver toxicity was first described in 1966, NAC was added as an antidote in 1974, and in 1975 came the nomogram with which the risk can be estimated from the blood level.

This is the clearest chain of action that any biohacking substance has: glutathione is used up, NAC replenishes the building block, and the effect depends measurably on how quickly this happens. The German antidote regimen gives 150 mg/kg in the first 60 minutes and a total of 300 mg/kg over 21 hours. Everything else borrows its credibility from here.

COPD: the second-best data set

The current meta-analysis of 14 RCTs with 2,856 patients arrives at a risk ratio of 0.87 for acute exacerbations and found no improvement in FEV1, FVC, quality of life or glutathione levels. The benefit concerns the flare-ups, not lung function. The studies contradict each other: BRONCUS missed both primary endpoints in 523 patients over three years, and a 2024 study in 968 patients with mild to moderate COPD missed both co-primary endpoints.

The Cochrane review of 38 studies with 10,377 participants puts the magnitude in perspective: eight people must be treated for nine months for one additional person to remain exacerbation-free, and newer studies show smaller effects than older ones. The German National Disease Management Guideline (Nationale VersorgungsLeitlinie) COPD therefore gives only an open recommendation for long-term therapy and states that there is no evidence for use in the acute situation.

Psychiatry and fertility

A different mechanism applies here: NAC acts on the glutamate balance. In depression, a meta-analysis of 12 RCTs with 904 patients found a small add-on effect of −0.24 in standardized mean difference. In trichotillomania, among 50 adults, 56 % were much or very much improved after 12 weeks versus 16 % on placebo — in 39 children, this could not be replicated. In obsessive-compulsive disorder, the meta-analysis rests on 6 RCTs with 195 patients and reached p = 0.05 only in the window of five to eight weeks. For craving in addiction, the largest study was negative: 302 adults with cannabis use disorder, odds ratio 1.00.

On male fertility, a Cochrane review pools 90 studies with 10,303 subfertile men and 20 antioxidants. The live birth rate showed an odds ratio of 1.43 with very low certainty; after excluding the studies at high risk of bias, nothing significant remained.

What happens to glutathione

This is where the most interesting gap lies. Oral absorption is low: in six healthy volunteers, bioavailability was 4.0 % for reduced NAC and 9.1 % for total NAC. Two findings dampen the glutathione claim: in an open-label study in eight people, brain glutathione did not rise significantly after 28 days at 6,000 mg daily, and the COPD meta-analysis found no change in glutathione levels.

The increase is established where there is a deficiency: in 18 patients with cystic fibrosis, NAC improved the glutathione deficiency of blood neutrophils over four weeks, without improving lung function. NAC fills up where something is missing. It does not generally raise levels where nothing is missing.

What the studies show

The US national multicenter program on paracetamol poisoning

Analysis of oral NAC administration from 1976 to 1985. Of 11,195 reported suspected cases, 2,540 treated patients were analyzed, with 140 mg/kg as a loading dose followed by 70 mg/kg every four hours for 17 further doses. Among at-risk patients, 6.1 % developed liver damage when treatment started within 10 hours, 26.4 % after 10 to 24 hours, and 41 % of high-risk patients after 16 to 24 hours. 11 patients died. Not a randomized comparison, but an analysis against historical controls.

PANTHEON: COPD, primary endpoint met

Randomized, double-blind, placebo-controlled trial at 34 hospitals. 1,006 patients with moderate to severe COPD, FEV1 between 30 and 70 % of predicted, for one year on 600 mg acetylcysteine twice daily or placebo. The primary endpoint was the annual exacerbation rate: 1.16 versus 1.49 per patient-year, risk ratio 0.78. Funded by the manufacturer; it allows no conclusion for mild COPD.

Brain glutathione under a high oral dose

Open-label, four-week study in eight people, five of them with Parkinson's disease, with 6,000 mg NAC daily; glutathione in the occipital cortex was measured by magnetic resonance spectroscopy. In the blood, catalase and the ratio of reduced to oxidized glutathione rose significantly, the markers of oxidative damage remained unchanged, and in the brain there was no increase. Very small sample, no placebo group.

Where the data stop

The core marketing claim is not established: that oral NAC raises glutathione levels in the liver, lungs and brain of healthy people. For the brain there is a negative finding; for the lungs, the COPD meta-analysis found no change. Also not established is the advice to take NAC on an empty stomach or with vitamin C.

Liver protection against alcohol is not established. The only randomized data come from severe alcoholic hepatitis: there, prednisolone plus NAC missed six-month mortality in 174 patients, at 27 % versus 38 %. In acute liver failure unrelated to paracetamol, intravenous NAC missed overall survival in 173 patients (70 % versus 66 %) but improved transplant-free survival (40 % versus 27 %), especially in early stages. This is a hospital treatment and no evidence of everyday liver protection. On detoxification and anti-aging in healthy people, randomized trials with clinical endpoints are lacking.

Pulmonary fibrosis is clearly negative: in the NAC arm of the PANTHER trial, with 133 versus 131 patients, forced vital capacity changed by −0.18 versus −0.19 liters over 60 weeks. And in COPD, quality of life in the Cochrane review missed the threshold of −4 points, at −1.37 points.

Status, approval and legal

In Germany, acetylcysteine is an approved medicine and not a food supplement. The oral preparations are approved for loosening mucus and easing expectoration in respiratory diseases with viscous mucus, available only in pharmacies and without prescription; the approved adult dose is 400 to 600 mg daily. The antidote is a separate, prescription-only preparation for poisoning with paracetamol, acrylonitrile, methacrylonitrile and methyl bromide. There is no maximum level from EFSA or the BfR, and no authorized health claim either. On August 1, 2022, the FDA determined that NAC is not a dietary supplement in the US either, but it exercises enforcement discretion there. Acetylcysteine is not on the 2026 WADA Prohibited List; under section M2.2, however, intravenous infusions of more than 100 mL per 12 hours are prohibited.

Safety

NAC is considered well tolerated, and the data on this are unusually broad. The most common side effects are gastrointestinal complaints: in the studies analyzed by the guideline, 6 % versus 5 % in the control groups. Use is contraindicated in children under 2 years because the loosened secretions can block the airways, and for 200 mg tablets additionally under 6 years. Caution is advised in asthma; bronchospasm is described as a rare side effect, and severe skin reactions have been reported, including Stevens-Johnson syndrome and Lyell syndrome. Three interactions matter: a gap of at least 2 hours from oral antibiotics, no combination with cough suppressants because of the risk of a build-up of secretions, and possible enhancement of the effect of nitroglycerin. The anaphylactoid reactions one reads about concern intravenous administration.

Sources

• Smilkstein et al., N Engl J Med 1988 (oral NAC in paracetamol overdose)
• Bateman et al., Lancet 2014 (shortened IV regimen, SNAP)
• Zheng et al., Lancet Respir Med 2014 (PANTHEON)
• Decramer et al., Lancet 2005 (BRONCUS)
• Zhou et al., Nat Commun 2024 (NAC in mild to moderate COPD)
• Cai et al., PeerJ 2026 (meta-analysis of NAC in COPD)
• Poole et al., Cochrane Database Syst Rev 2019 (mucolytics)
• Peng et al., Gen Hosp Psychiatry 2024 (meta-analysis, depression)
• de Ligny et al., Cochrane Database Syst Rev 2022 (antioxidants for male subfertility)
• Coles et al., J Clin Pharmacol 2018 (brain glutathione)