N-acetylcysteine is the acetylated form of the amino acid cysteine, and the body uses it to make glutathione, its main antioxidant. It has been a hospital medicine for decades: intravenously as the antidote to paracetamol poisoning, and orally or by inhalation to thin mucus in lung disease.

Its supplement career rests on the glutathione link. If oxidative stress is involved in a condition, the reasoning goes, a glutathione precursor might help. That reasoning has been tested in trials for lung disease, where it holds up reasonably, and across psychiatry, where the results are mixed and the enthusiasm has mostly outrun them.

NAC supplies cysteine, the rate-limiting ingredient for glutathione synthesis, and glutathione is the molecule cells use to neutralize reactive oxygen species and to detoxify drugs, which is why NAC rescues the liver in paracetamol overdose. In the airway its free thiol group breaks the bonds that make mucus thick, the mucolytic effect.

In the brain NAC also modulates glutamate through a cysteine-glutamate exchanger, which is the proposed route to effects on compulsive behavior, addiction and psychosis. That mechanism is plausible and the trials testing it are small.

Chronic bronchitis and COPD: consistent. A 2015 meta-analysis of 13 studies in 4,155 patients found NAC reduced exacerbations by about a quarter, with the effect clearer in patients without airway obstruction and higher doses needed for those with it. A 2024 pair of meta-analyses of 20 studies found fewer exacerbations in both COPD and chronic bronchitis, and a higher likelihood of symptom and quality of life improvement in the latter.

Psychiatry: it depends on the disorder. A 2018 meta-analysis of six trials in 701 people found adjunctive NAC improved total symptoms in schizophrenia, had no significant effect on manic or depressive symptoms in bipolar disorder, and only a small effect in major depression. A 2016 meta-analysis of five trials in 574 people with depressive symptoms across conditions found a modest improvement in depression scores and functioning, with minor adverse events. Those two reviews overlap in their trials and differ in their verdict, which is the state of the field.

Everything else. Hangovers, fertility, liver protection in people without poisoning, exercise recovery: small trials or none.

Oral NAC is well tolerated, with nausea, vomiting and diarrhea the common effects and a sulfur smell to the capsules. Meta-analyses in lung disease found adverse reactions no more common on NAC than placebo. The intravenous antidote can cause allergic-type reactions, which is a hospital matter and irrelevant to capsules.

The theoretical concern is that a strong antioxidant could blunt processes that depend on oxidative signaling, including some adaptations to exercise and possibly some cancer treatments. That is a reason for people on chemotherapy to ask before taking it, not a demonstrated harm.

Acetylcysteine is a licensed prescription medicine in the United States, the United Kingdom and the European Union for paracetamol overdose and as a mucolytic. In the United States the FDA stated in 2020 that NAC was excluded from the dietary supplement definition because it was approved as a drug first, then in 2022 announced it would not enforce that position while it considered a rulemaking, which is why NAC supplements are back on shelves with their status unresolved. In Europe it is sold over the counter as a cough medicine in several countries.