NAC (N-acetylcysteine) is a form of the amino acid cysteine that the body uses to make glutathione, a major antioxidant. It is an FDA-approved drug used as the antidote for acetaminophen (Tylenol) overdose and, inhaled, to thin thick mucus. It is also sold in capsules labeled as a supplement and studied for many other uses.
Also known as: N-acetylcysteine, N-acetyl-L-cysteine, acetylcysteine.
NAC has no intake value or upper limit. In the pharmacokinetic studies cited below, healthy volunteers took single doses of 400 mg (Olsson 1988) and 600 mg (Borgström 1986) by mouth. The amounts used as an overdose antidote are much larger and weight-based, given under medical care. These are study protocols, not a recommendation.
Acetylcysteine is FDA approved as Acetadote (by vein) to prevent or lessen liver injury after an acetaminophen overdose, and as an acetylcysteine solution that is inhaled as a mucus thinner or taken by mouth as the acetaminophen antidote. The oral solution label lists nausea, vomiting and other stomach symptoms with the large antidote doses, and rarely rash.
FDA has determined that NAC is excluded from the legal definition of a dietary supplement because it was approved as a drug before it was sold as a supplement. In an August 2022 final guidance, FDA said it intends to use enforcement discretion for products containing NAC that are labeled as dietary supplements and otherwise follow the law, unless it finds safety concerns, while it considers a rule that would allow NAC in supplements. So NAC capsules remain on sale, but under an FDA policy rather than as an ordinary supplement.
A 2019 review (Šalamon) describes NAC as considered safe, used as a drug since the 1960s and on the WHO list of essential medicines as a poisoning antidote, and notes that results of trials for other uses are sometimes conflicting or incomplete.
Studies disagree depending on what is measured. In 6 healthy volunteers (Olsson 1988), the half-life of total NAC was 6.25 hours after 400 mg by mouth, while free (reduced) NAC had a half-life of 1.95 hours after a dose by vein; only 4% (reduced) to 9.1% (total) of an oral dose reached the blood. In 10 volunteers given 600 mg (Borgström 1986), the late elimination half-life was 2.27 hours and oral availability 6% to 10%. Because the figures differ this much, we do not list one number.
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