NAC (N-acetylcysteine)

Is NAC safe?

For most healthy adults taking a label dose, yes. N-acetylcysteine (NAC) has been an FDA-approved drug since 1963, first as an inhaled mucus thinner and later as the antidote that protects the liver after an acetaminophen overdose. As a supplement it is usually sold in 500 or 600 mg capsules, taken up to about 1,800 mg a day.

Three things matter more than the safety record. NAC upsets some stomachs. It interacts with nitroglycerin. And in the US it is not, legally, a dietary supplement at all: FDA allows it to be sold that way by choice, not by right.

What are the side effects of NAC?

Mostly digestive. The oral drug label lists nausea, vomiting and other stomach symptoms, "especially in the large doses needed to treat acetaminophen overdose", plus rare rash and hives. Many users also notice the sulphur smell of the capsules.

At supplement doses the picture is milder. A 2017 meta-analysis in Sports Medicine pooled 17 studies and found side effects only slightly and not significantly more common than with placebo (odds ratio 1.11). The same review saw more side effects as doses rose; athletes in some studies took several grams a day. A 2015 review of psychiatric and neurological trials, which often used 2,000 to 3,000 mg a day for months, concluded that NAC "appears to be safe and tolerable".

Liver safety is the strongest point. LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary, the US government database of drug-induced liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, says acetylcysteine has not been linked to significant enzyme rises or to clinically apparent liver injury.

Why FDA says NAC is not a supplement

US law excludes from the supplement definition any substance approved as a drug before it was sold as a supplement or food. FDA's August 2022 guidance sets out the dates: acetylcysteine was approved as a new drug on 14 September 1963, and FDA knows of no supplement or food sales before then.

FDA began citing that rule against NAC products in 2020, although they had been sold as supplements for about 30 years. The Council for Responsible Nutrition and the Natural Products Association petitioned in 2021. On 31 March 2022 FDA denied both petitions but did not rule on the Natural Products Association's alternative request for a regulation allowing NAC in supplements.

Then came the compromise. In the August 2022 guidance, FDA said its review so far had shown no safety concerns and that it would not object to NAC supplements that make only structure and function claims. The tolerance does not cover products that claim to treat or prevent a disease, and it lasts until FDA either completes rulemaking or denies the request. We found no later FDA decision on FDA's site, in the Federal Register or in the petition docket, so as of October 2026 the 2022 policy is the current one.

For buyers, the practical effect is small: NAC stays widely available. But a seller promising that NAC treats COVID-19, addiction or lung disease is outside FDA's tolerance as well as the evidence.

Interactions and who should be careful

The best-documented interaction is with nitroglycerin. In a 1992 study, 100 mg/kg of NAC made nitroglycerin headaches worse and widened arteries further in healthy volunteers, and NAC has been studied as a way to boost nitrate effects in angina. If you use nitroglycerin or another nitrate, talk to your doctor before adding NAC.

Other groups to check with a doctor or pharmacist first:

• People with asthma. Inhaled acetylcysteine can trigger bronchospasm, and the label asks for careful monitoring. Oral NAC rarely does this, but it is worth knowing.
• Pregnant and breastfeeding women. NAC is used in pregnancy when a mother has overdosed on acetaminophen, but long-term supplement use has not been studied.
• Anyone treated for an overdose. Supplement capsules are no substitute for hospital treatment. The oral antidote regimen starts at 140 mg per kg of body weight, about 9.8 g for a 70 kg adult, and is guided by blood tests.

Is NAC quality reliable?

Probably better than for herbs, but it is unverified. NAC is a single synthetic molecule made to pharmaceutical standards, so heavy-metal and plant-adulteration problems that dog botanical products are less likely. The openFDA enforcement database lists no recalls of NAC supplements. We found no published independent test of NAC content in US products, though, and NAC slowly oxidises in air, so check expiry dates and keep the container closed.

How to use NAC more carefully

1. Stay at label doses, usually 600 to 1,800 mg a day, and take it with food if it upsets your stomach.
2. Ignore disease claims. A product sold to treat a specific illness is outside FDA's 2022 tolerance.
3. Prefer USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary or NSF-certified products where available, since no other quality check exists.
4. Tell your pharmacist if you take nitrates, and see the NMN page for another supplement caught by the same drug-first rule.

How we scored it

Organ toxicity (90) is the strongest factor: no liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary in LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary and decades of use as a liver-protecting antidote. Dose safety (82) reflects mild but common stomach effects. Interactions (78) are limited, with nitroglycerin the main one. Quality (70) is unproven rather than poor, and regulatory actions (58) reflect a US status that depends on FDA enforcement discretion.