NAC

Acetylcysteine is registered in Spain as a medicine, not as a food supplement: it is authorized as a mucolytic and as a hospital antidote for paracetamol poisoning. Those medical uses are outside the scope of this page.

This page describes the regulatory status in Spain and the EU. Acetylcysteine is not regulated the same way everywhere, so if you are reading from another country, check the rules that apply where you live.

What you will find here is what research has measured when NAC has been studied as a supplement: in which populations, at what doses, with what result and with what degree of certainty. Suplepedia describes what published studies say and does not recommend taking it.

If you are considering taking NAC, discuss it with your doctor or pharmacist. If you have asthma or a history of bronchospasm, that consultation is especially relevant: it is the only NAC safety signal with direct regulatory backing, and it is detailed in Precautions.

NAC is a modified version of cysteine, one of the amino acids you already get from food. The added acetyl group makes it more stable and easier to absorb than free cysteine.

Inside the cell, cysteine is the part in short supply for making glutathione, the main antioxidant the body produces on its own. Supplying cysteine makes it possible to make more glutathione. That is the whole mechanism, and every promise surrounding NAC comes out of it.

NAC was not born as a supplement. The FDA approved it in 1963 as an inhaled mucolytic, taking advantage of the fact that its thiol group breaks the disulfide bridges that thicken mucus. Its second life began in Edinburgh: in 1977, Laurie Prescott and his team published in The Lancet that intravenous acetylcysteine protected the liver almost completely against a paracetamol overdose if it was given early. Since then it has appeared on the WHO Model List of Essential Medicines, in the antidotes chapter.

Glutathione, however, does not explain everything. There is a second pathway that has nothing to do with antioxidants and that accounts for a good part of the published trials. NAC acts on the cystine/glutamate exchanger, a gate in the cell membrane that brings cystine in exchange for sending glutamate out. In doing so it alters the amount of glutamate available between neurons, and it is that pathway that is invoked in the trials on repetitive behaviors and addictions.

Out of that come three documented actions and one hypothesis. You already know the first: it supplies cysteine, the limiting amino acid in glutathione synthesis. The second is a direct antioxidant action, recognized in the product information itself: the sulfhydryl group donates electrons to the electrophilic groups of oxidizing radicals without going through glutathione. The third is the mucolytic effect, by depolymerization of the mucoprotein complexes and breaking of the disulfide bridges that hold mucin together.

The open hypothesis suggests that NAC works less as an antioxidant than is assumed. Recent mechanistic literature proposes that inside the cell it is converted into hydrogen sulfide and sulfane species, which act as signaling molecules. If this were confirmed, it would force the whole framework to be rewritten: it would not be a shield that absorbs radicals, but a messenger. This is work in cells and animals, without human confirmation.

Cysteine is synthesized from methionine by the transsulfuration pathway, with vitamin B6 as a cofactor.

Acetylcysteine is used in Spain as a medicine in two authorized indications: as a mucolytic and as an antidote for paracetamol poisoning in the hospital setting. These are medical uses, with their own product information, their own doses and their own clinical supervision. We do not develop them here.

Official information on each product authorized in Spain is published by the AEMPS in its CIMA database.

In athletic performance, the Australian Institute of Sport places it in Group B: emerging evidence, use only under supervision in research or in scenarios managed case by case. The trials with oral capsules contradict one another. The only pattern that orders them is that the effect shows up in those who started with low glutathione, and not in those whose level was normal.

In aging and oxidative stress, the results usually cited come from GlyNAC, a combination of glycine and NAC that is not the same thing as NAC alone. Its own researchers hold that NAC by itself does not reproduce those results.

In cognition, no trial has been found showing an improvement in memory, attention or mental speed in healthy people.

Studies in patients, not in healthy people. NAC has also been tested in people with specific diagnoses, from trichotillomania to schizophrenia, in many cases added to their usual treatment and under medical supervision. The results are mixed. It is not an indication, it is not a regimen and it is not a reason to take NAC on your own.

During muscle contraction, reactive oxygen species surge. In excess they contribute to fatigue, but in physiological amounts they are the signal that orders the body to adapt: to make more mitochondria, to grow. NAC entered sport through the first half of that sentence and the whole discussion since then turns on the second.

The classic line of research was intravenous. An Australian group published two trials in 2004 with the same infusion protocol: in the first, with seven trained men, time to fatigue did not increase significantly; in the second, with endurance athletes, it increased by 26%. That 26% has been cited for twenty years as proof that NAC delays fatigue, frequently attributed to the wrong trial. The concentrations an infusion reaches are not reproduced by a capsule, which delivers a small fraction of the dose to the blood.

NAC does not appear on the World Anti-Doping Agency’s 2026 Prohibited List, in force since 1 January. The list is replaced every year and is not exhaustive: under the principle of strict liability, the registered athlete is responsible for what is in their body, and checking a specific substance is done on GlobalDRO, not on an informational page. The Australian Institute of Sport expressly advises against continued use beyond one month.

The anti-aging promise does not rest on NAC, but on GlyNAC, the combination of glycine and N-acetylcysteine. The people who explain it best are the group that developed it: they hold that glycine is the part that limits glutathione production, and they review the human trials with NAC alone that failed to raise glutathione or reduce oxidative stress. Their hypothesis is that NAC alone, in making glutathione, consumes the available glycine instead of supplying it.

An independent trial with 114 healthy older adults adds another layer. There GlyNAC did not raise circulating glutathione against placebo, which was the primary outcome; the increase appeared only in the subgroup that started with low glutathione and high oxidative stress. Two trials of the same combination disagree on the most basic measure of all.

The benefits, moreover, do not stay. In the same group’s earlier pilot trial, twelve weeks after GlyNAC was withdrawn, glutathione and the oxidative stress markers had gone back almost to their starting values.

On infections, the reference trial was published in 1997 with 262 participants, most of them over 65, treated for six months. NAC did not prevent people from catching flu, but among those infected, symptoms developed in 25% of those taking it against 79% of the placebo group. It was funded by an Italian acetylcysteine manufacturer, which supplied the coded tablets.

The Alzheimer’s Drug Discovery Foundation, through its Cognitive Vitality program, rates the evidence for NAC against cognitive decline with a D, its grade for the lack of quality randomized trials. Its assessment located eight randomized trials and none that examined the prevention of cognitive decline or dementia with NAC alone. In probable Alzheimer’s, the only trial with NAC as monotherapy did not move the primary measure. In mild cognitive impairment of vascular origin, 24 weeks likewise added nothing to rehabilitation. The most striking human result comes from a trial with 81 military personnel with mild blast-induced head injury. In the secondary analysis of the subgroup treated within the first 24 hours, 86% were free of symptoms by the seventh day against 42% on placebo; started later, there was no effect. This is acute neuroprotection after an injury, not the mental performance of a healthy person.

The reactive oxygen species that contracting muscle produces oxidize contractile proteins and blunt the activity of the sodium-potassium pump, two specific mechanisms of fatigue. That is NAC’s theoretical target. The same oxidative signal activates mitochondrial biogenesis via PGC-1α, and that is where the conflict lies.

The first intravenous trial of 2004 deserves a close reading, because its misquotation has sustained twenty years of claims. Its primary outcome, time to fatigue, was negative: 14.6 minutes on average with NAC against 12.8 with control, without statistical significance. What it did find was a correlation between the percentage change in time to fatigue and each subject’s peak VO₂, r=0.78 with p<0.05. It is a secondary analysis on seven people, and out of it came the idea that the response depends on training status. The 26.3% increase belongs to the second trial, done with endurance athletes.

By the oral route, the acute-dose trial that tested four intensities found an improvement in time to fatigue only at 80% of peak power; at 90, 100 and 110% there was no effect, and work capacity above critical power tended downward. The pattern suggests that, if NAC does anything, it does it in prolonged submaximal fatigue and not in short maximal effort.

A later piece of work from the same group as the screening trial reinforced the deficit hypothesis from another angle. It screened 73 volunteers by their plasma vitamin C and their erythrocyte glutathione and assigned them supplements aimed at their own shortfall or unrelated to it: the ergogenic effect appeared only when the antioxidant reversed that particular person’s deficiency. Its authors conclude that the presence of oxidative stress is not enough to justify supplementing, and that it is necessary to identify in advance who responds. The practical implication is uncomfortable for the consumer, because it requires knowing what you are starting from, and there is no standardized way of knowing it.

The signal of interference with adaptations is not unanimous either. Reviews of the field point out that three studies on sarcohormesis in athletes did not confirm the blunting effect attributed to NAC. The strongest evidence of interference comes from trials with vitamins C and E, not with NAC, and is used as a conceptual framework, not as direct proof. There is no prolonged training trial with oral NAC that measures real gains in strength, hypertrophy or performance at the end of the period. That is the central gap of the sports axis, and without it neither the long-term benefit nor the long-term harm can be asserted.

Almost all the sports research with NAC has been done in men, with samples of between seven and fifteen participants.

The two most cited positive trials share a research group and a territory: body-focused repetitive behaviors. In 50 adults with trichotillomania, NAC beat placebo on the two scales specific to the disorder, with benefit from the sixth week onward. The one on excoriation disorder, with 66 adults, is detailed in the studies section. The replication in 39 children and adolescents aged 8 to 17 with trichotillomania, with NAC added to their treatment, was negative: no significant difference against placebo on any measure, primary or secondary.

Outside that territory the results deteriorate as the trials grow. In 252 patients with major depression, the group-by-visit interaction on the main scale was not significant at twelve weeks, although at sixteen differences appeared in secondary outcomes. In bipolar disorder, a maintenance trial with 149 participants found no difference in time to the next episode, with the particularity that all of them had received open-label NAC before randomization, a withdrawal design that may have masked the effect. In 84 patients with clozapine-resistant schizophrenia, a year of treatment did not improve negative symptoms, cognition or quality of life. The phase III trial in obsessive-compulsive disorder, with 98 participants and twenty weeks, was negative on the primary outcome and on every secondary one; the mean difference on the main scale pointed slightly in favor of placebo.

A trial in early psychosis brings in the same nuance that appeared in sport: over six months, the improvement in processing speed and in positive symptoms appeared only in the subgroup with raised peripheral oxidative stress, together with a 23% increase in glutathione in the prefrontal cortex measured by spectroscopy. The trial used 2,700 mg a day. The effect is once again conditional on the starting state.

The positive trials are concentrated between 2009 and 2016 and the negative ones are the recent, the large and the multicenter ones. It is the usual pattern of a field whose first signals do not survive replication.

In the respiratory field, which is that of the medicine and not that of the supplement, the results are not homogeneous either. The notice above applies in full to the three paragraphs that follow.

In 70 people with cystic fibrosis, 24 weeks of treatment did not reduce neutrophil elastase activity in sputum, which was the primary outcome; a secondary lung function outcome improved by around 150 mL of FEV₁. In COPD, a three-year trial with 523 patients did not reduce the decline in lung function or the exacerbations except in the subgroup that was not using inhaled corticosteroids. Another, with 1,006 Chinese patients with moderate and severe disease treated for a year, did reduce the exacerbations, above all in those of moderate severity. The most recent and largest, with 968 patients with mild and moderate COPD over two years, was negative on both outcomes, and used the same daily dose of 1,200 mg as the earlier positive trial: what separates the two is not the amount, but the severity of the disease. In asthma there is no trial showing benefit, and there is a safety precaution.

In autoimmunity there is a single positive trial that meets the criteria, and it is a pilot. In 36 patients with systemic lupus erythematosus, three different amounts were compared against placebo, between 1.2 and 4.8 grams a day. The two highest arms reduced disease activity and fatigue over four months; the lowest did not. The authors themselves describe it as a proof of concept that requires confirmation.

The counterpoint is a trial in 74 patients with rheumatoid arthritis. Against placebo, twelve weeks of treatment did not improve disease activity or the markers that measured inflammation and oxidative damage: not the DAS-28 index, not malondialdehyde, not total antioxidant capacity, not glutathione peroxidase, not high-sensitivity C-reactive protein. Nitric oxide and blood glucose did fall and HDL rose.

The distinction matters, because it is what separates a trial from an impression. Comparing the two groups with each other is what makes it possible to attribute a change to the treatment; looking at how each group changed on the inside does not. A methodological comment published in the same journal points out that the NAC group started with malondialdehyde at almost double that of the placebo group, 4.24 against 2.21 micromolar, and that its later 65% fall within that group, highly significant seen in isolation, says nothing while the comparison between groups still shows no difference.

On inflammatory markers, a meta-analysis of 24 trials with 1,057 participants found that oral NAC reduces C-reactive protein and interleukin 6, but does not change TNF-α; the reduction in TNF-α appears only with intravenous administration. The commercial claim that NAC inhibits TNF-α is, for a capsule, incorrect.

The randomized GlyNAC trial is worth reading for its negatives as much as for its positives. Alongside the improvements in glutathione, oxidative stress, inflammation and insulin resistance, there was no change in fat mass, in fasting blood glucose, in the glucose tolerance test, in the telomeric marker POT1 or in perceived quality of life, which the authors themselves point out as an unexpected result. The reduction in body fat that circulates in popular coverage comes from the open pilot trial without placebo that preceded the randomized one, and was not replicated in it: there only waist circumference fell.

The population is not just anyone either. Adults aged 61 to 80 with a body mass index above 27 were selected, and the criterion was set deliberately because the rodent studies had shown weight loss. There is no GlyNAC trial in middle-aged adults or in people without excess weight.

The same authors bring in a limit that is rarely cited: reductive stress. Cells need a minimum amount of reactive species in order to signal, and suppressing them to excess also does damage. They point out that excessive administration of NAC alone or of glutathione accelerates aging in the nematode Caenorhabditis elegans, and argue that their intervention avoids this because the oxidative markers fall to the values of the young group and not below them. It is a reasonable argument and, along the way, an implicit warning about high doses of NAC alone.

In metabolism the signal is consistent but narrow. In 76 people with metabolic syndrome, twelve weeks of treatment reduced fasting blood glucose, insulin and HOMA-IR. The trial used 1,800 mg a day. In another with 69 patients with metabolic fatty liver disease, NAC did not touch the steatosis or the transaminases but it did reduce blood glucose, insulin, HOMA-IR and C-reactive protein, and raised total glutathione. The effect that shows up repeatedly is on insulin resistance, not on the organ or on perceived energy: a pilot trial of fatigue in progressive multiple sclerosis was negative.

These are the amounts the trials used, each one with its own population and its own context, not a regimen for use. The Australian Institute of Sport, which reports the figure of 1,200 mg, recommends taking it with food to reduce digestive discomfort; it is advice about tolerance, not about absorption, and no study has been located that measures how much food affects the absorption of NAC.

Of an oral dose of NAC, between 6 and 10% reaches the blood intact. The rest is metabolized in the intestinal wall and in the liver before it circulates. The maximum concentration is reached between forty minutes and three hours depending on the source consulted, and the half-life is around six hours. Hence the intravenous trials say nothing about capsules, and hence too that penetration into the brain is limited.

Absolute oral bioavailability has been estimated at 11.6%, with a 95% confidence interval between 6.3 and 16.9%, in critically ill patients. After 600 mg orally, the time to maximum concentration is around 40 minutes according to some product information sheets and two to three hours according to others in patients with respiratory or cardiac disease, with a maximum concentration of free acetylcysteine of around 15 nmol/mL and a half-life of approximately six hours. The variability between sources is unresolved.

No alternative form has shown superiority on a clinical outcome. The bioavailability figures for NACA come from animal models.

On the effervescent formulation, no study has been located that compares its absorption with the standard one. The sustained-release forms have been characterized pharmacokinetically in clinical populations, without performance trials. N-acetylcysteine amide was designed to increase lipophilicity and membrane permeability, and in mice it reached 67% bioavailability against 15% for NAC, with a capacity to replenish glutathione three or four times greater; there is no human trial at all in sport or in cognition.

No quality seals or batch-by-batch testing programs specific to NAC have been located, of the kind that exist for other supplements. Bought in a pharmacy as a medicine, on the other hand, the package is subject to the pharmaceutical control of identity, content and batch, which is a different guarantee and not an equivalent one.

In Spain and the European Union, acetylcysteine is registered as a medicine, with ATC code R05CB01, and cannot be marketed as a food supplement.

In the United States the status has been in dispute for years. In 2020 the FDA sent warning letters to companies that were selling NAC as a dietary supplement, holding that it is excluded from that definition because it was approved as a drug before it was marketed as a supplement. Two industry associations filed citizen petitions in 2021 and one of them sued the agency. In March 2022 the FDA refused to reconsider the exclusion, and in August it published final guidance on enforcement discretion: it will not pursue the sale of products that would be lawful supplements if NAC were not excluded, provided they are not presented as diagnosing, curing, treating or preventing disease. The legal exclusion remains in force; what changes is that it is not enforced.

The practical consequence for whoever buys is that a European pharmacy package is subject to control of identity, content and batch as a medicine, whereas a bottle bought under the United States framework is governed by rules under which the manufacturer does not have to prove efficacy or safety before selling. No independent analysis of declared content against actual content limited to NAC products has been located, so on the reliability of its labeling there are no public data: it is a gap, not a guarantee.

How to know whether you are starting from a shortfall. The finding that orders the sports evidence depends on measuring baseline glutathione, and there is no standardized biomarker or agreed threshold that makes it possible to do so outside a laboratory.

Whether the oncological signal in animals means anything in people. There are no human trials that confirm it or rule it out.

What happens in the long term and on stopping. Almost all the trials last weeks or a few months.

How women respond. The sports research has been done almost entirely in men.

Absence of a dose-response curve. The doses used in the trials run from 600 mg to almost 5 grams a day without it having been characterized where the effect starts or where it stops increasing.

Lack of direct comparisons. There are no trials comparing oral NAC against intravenous, or one formulation against another, in the same protocol and with the same outcomes.

The most frequent adverse effects are digestive: nausea, vomiting, diarrhea and abdominal pain. The molecule’s sulfur smell does not help either.

The chelation of minerals that is cited to justify separating doses is theoretical. In ten healthy volunteers who took 600 mg a day for two weeks, neither the plasma concentrations nor the urinary excretion of calcium, magnesium, iron, zinc or copper changed. The warning about histamine intolerance that appears in the product information does not have clinical trial backing yet either: its only experimental support is an in vitro conference abstract whose author was linked, at the time it was published, to a manufacturer of diagnostic tests for the enzyme involved.

Bronchospasm. The AEMPS product information indicates that use should be assessed carefully in people with asthma or a history of bronchospasm, because it can increase the obstruction of the airway, above all by the inhaled route, and that treatment should be stopped if it appears. Fatal anaphylactoid reactions have been described with intravenous acetylcysteine in unstable asthma. By the oral route no clear cases of bronchospasm have been reported, but the regulatory precaution stands and the psychiatric trials systematically exclude people with asthma.

Preclinical oncological signal. In murine models of induced lung cancer, supplementing the diet with NAC and vitamin E markedly increased tumor progression and reduced survival. In an endogenous melanoma model, NAC increased metastases in lymph nodes without changing the number or the size of the primary tumor, with a higher ratio of reduced to oxidized glutathione in the metastases. In human melanoma cells in vitro, NAC and a vitamin E analogue increased migration and invasion. These are preclinical data and they do not prove that the same thing happens in people, but there is no human trial that confirms it or rules it out.

Interactions. It can potentiate the vasodilator effect of nitroglycerin, with headache and hypotension. Its possible chelating effect appears in the product information as a reason to separate the intake from gold, calcium and iron salts, although the only trial that measured it in people found no changes. It should not be combined with cough suppressants or with inhibitors of bronchial secretions, because it would hinder the elimination of the thinned mucus. There is a precaution in peptic ulcer or a history of it.

Pediatric population. Mucolytics, acetylcysteine among them, are contraindicated in children under two because of the risk of respiratory obstruction.