NAC supplement (N-acetyl cysteine): medicine or supplement, the evidence and who should avoid it

What NAC (N-acetyl cysteine) is

NAC stands for N-acetylcysteine, a modified form of the amino acid cysteine. Your body uses it to make glutathione, one of the cell’s own main antioxidants. Acetylcysteine also breaks down thick mucus so it’s easier to cough up.

In much of Europe, acetylcysteine is a familiar pharmacy medicine for chesty coughs, sold as effervescent tablets and granules. In the UK it’s prescription-only. On iHerb the same substance comes as capsules labelled NAC, usually 600 mg each.

Medicine or supplement: NAC’s legal status

In most countries NAC is a medicine, not a food supplement. It’s sold as a supplement mainly in the US.

The US story. Acetylcysteine was approved as a drug in 1963. Under US law, a substance approved as a drug first can’t later be sold as a supplement. In July 2020 the FDA pointed this out in warning letters to companies selling NAC as a hangover remedy. Industry groups petitioned. In August 2022 the FDA issued final guidance: it won’t object to NAC labelled as a supplement if the product makes no disease claims and is otherwise lawful.

What this means for you:

• NAC from iHerb is a medicine ingredient in a capsule, just without a patient leaflet.
• Check at checkout whether the product can be delivered to your country; rules for medicine ingredients change.
• In the UK, licensed oral acetylcysteine needs a prescription, so your GP is the person to ask whether you need it at all.

NAC benefits: what’s proven and what isn’t

So acetylcysteine has proven medical uses, but almost all of them involve a doctor. For a healthy person taking it “just in case”, no benefit has been shown.

On COVID-19, NIH ODS reviews several studies. Small studies linked NAC at 600 mg twice a day with better outcomes. Two studies of intravenous acetylcysteine in hospital found no effect on deaths or the need for ventilation. NIH ODS concludes there isn’t enough data.

Pick your reason for wanting NAC to see what’s known and who should make the call.

Safety check before you buy

If you’re still considering NAC, run through this short check. Each line covers a real risk.

NAC dosage

Most NAC capsules on iHerb contain 600 mg. In the COVID-19 studies cited by NIH ODS, adults took 600 mg twice a day for 5 to 14 days. UK product information for the licensed 600 mg effervescent tablet gives adults one tablet once a day to loosen mucus.

Practical rules:

• Don’t exceed the dose on the label or take NAC for months without medical advice.
• Swallow it with a full glass of water, and take it with food if it makes you feel sick.
• Don’t combine it with cough suppressants: they stop you coughing up the loosened mucus.
• Take oral antibiotics 2 hours before or after acetylcysteine, as UK product information advises.

NAC side effects

According to NIH ODS, the most common effects when taken by mouth are nausea, vomiting, stomach pain, diarrhoea, indigestion and upper abdominal discomfort. Many people notice a sulphur smell when they open the tub. That’s a property of the substance, not a sign it has gone off.

Allergic reactions are rare: rash, itching, swelling or difficulty breathing. If they happen, stop taking it and get medical help.

Who should avoid NAC, and interactions

Avoid these combinations unless a doctor advises otherwise.

• GTN (glyceryl trinitrate). NIH ODS warns that with NAC it can cause low blood pressure and severe headaches. If you take any other angina medicine, ask your pharmacist.
• Blood pressure medicines. NIH ODS says NAC may lower blood pressure, adding to their effect.
• Anticoagulants (warfarin and other blood thinners). NAC may slow blood clotting, so the effects could add up.
• Cough suppressants. They stop the cough reflex, so loosened mucus builds up.

In these situations, a doctor should decide.

• Asthma. Bronchospasm can occur, especially in people with asthma. Only on medical advice.
• Stomach or duodenal ulcer, including in the past.
• Pregnancy and breastfeeding. There’s little safety data for NAC supplements.
• Children. Only if a doctor prescribes it. The UK 600 mg tablet is licensed for adults only and must not be given under the age of 2. See our guide to vitamins for kids for what is recommended.

You can check other combinations in our vitamin compatibility chart.

Why NAC attracts so many promises

Much of the marketing rests on glutathione. NAC supplies cysteine, which cells use to make glutathione, so it’s sold for “antioxidant support”, “liver detox” and immunity. The mechanism matters in one setting: in paracetamol poisoning, the liver runs out of glutathione and acetylcysteine helps restore it.

In a healthy person whose glutathione isn’t depleted, there’s no evidence that extra NAC improves health. Antioxidant effects seen in lab dishes often don’t translate into benefits in people. So treat “supports detox” on a label as marketing, not as a proven effect.

If your goal is immunity

NAC is often placed in “immune support” ranges, but there’s no evidence that it protects healthy people from infections. If you keep getting ill, start with the basics: enough vitamin D, adequate zinc in your diet, sleep and the vaccines you’re offered. For what’s actually been studied, see our immunity section.

How to choose NAC on iHerb

1. Make sure you have a clear reason and your GP or pharmacist agrees. You don’t need NAC for “detox”.
2. Run through the check above: GTN, blood pressure medicines, blood thinners, asthma, ulcers.
3. Choose 600 mg or less per capsule and stick to the dose on the label.
4. Confirm the product can be delivered to the UK or your EU country.
5. If you need a mucolytic, ask your GP. In the UK licensed acetylcysteine is prescription-only and comes with a leaflet and a clear dose.

For reading doses per serving, see our Supplement Facts guide.