What Is NAC (N-Acetyl Cysteine)? Uses, Dosage and Safety in Supplements
Quick answer: What is NAC? N-acetyl cysteine (NAC) is a modified form of the amino acid cysteine. In the UK it is the active substance in some licensed medicines, where it is called acetylcysteine, and it is also sold in food supplements. No health claims are authorised for NAC in Great Britain, so supplement labels can state the amount but not a health benefit.
- A form of cysteine. The body converts NAC into L-cysteine, an amino acid found in protein.
- Also a medicine. As acetylcysteine, it is licensed in the UK for medical use.
- No authorised health claims. Claims proposed for NAC are listed as not authorised on the GB register.
- Amounts vary widely. A Dutch assessment set 1,200 mg a day as an adult reference value.
- Not for everyone. Ask first if you have asthma, a stomach ulcer or take medicines.
01What is NAC?
N-acetyl cysteine (also written N-acetylcysteine) is the N-acetyl form of L-cysteine, one of the amino acids that make up protein. In the body, NAC is converted into L-cysteine, which can in turn be converted to and from L-cystine.
Cysteine is already part of a normal diet as a building block of food proteins, and L-cysteine is an approved food additive (E920) in some flour products.
| Full name | N-acetyl-L-cysteine (NAC); called acetylcysteine as a medicine |
|---|---|
| What it is | The N-acetyl form of the amino acid L-cysteine |
| In the body | Converted into L-cysteine (and L-cystine) |
| Amounts in supplements | 200 to 2,000 mg a day on the Dutch market (Dutch National Institute for Public Health and the Environment, RIVM, 2024) |
| Adult reference value | 1,200 mg a day (VKM, 2015; adopted by RIVM, 2024) |
| GB health claims | None authorised |
| Reported side effects | Unpleasant taste, nausea, vomiting and diarrhoea (mainly from medicine use) |
02How is NAC used?
As a licensed medicine
Acetylcysteine is a licensed medicine in the UK. The British National Formulary (BNF) lists several forms, including effervescent tablets licensed for adults with certain long-term respiratory conditions, and an intravenous form that hospitals give after a paracetamol overdose.
These medicines are prescribed or given by healthcare professionals, and are a different category of product from a food supplement.
In food supplements
NAC is widely sold in capsules and powders. Health claims have been submitted for NAC, for example about glutathione, but the Great Britain nutrition and health claims (NHC) register lists them as not authorised.
In Great Britain, a NAC supplement can tell you how much NAC it contains, but not what it does for your health.
03What is NAC’s status in the UK?
Food supplements in the UK are regulated as food. GOV.UK guidance describes them as products intended to supplement the normal diet, and says they are not medicinal products.
Because acetylcysteine is also a licensed medicine, NAC sits close to the line between food and medicine. The MHRA decides whether a product counts as a medicine, looking at how it is presented (including any claims) and how it works. We are not aware of a GB-specific maximum amount for NAC in food supplements, and the position may change.
Great Britain
- Food law
- MHRA borderline rules
Sold as a food supplement ingredient, with no authorised health claims. A product presented as having medicinal properties would be regulated as a medicine.
The Netherlands (RIVM, 2024)
- Government risk assessment
- Reference values by age
Set reference values by age, advised against use in children under 2, and recommended not combining NAC supplements with NAC medicines.
United States (FDA, 2022)
- Final guidance
- Enforcement discretion
NAC is excluded from the legal definition of a dietary supplement because it was approved as a drug first, but the FDA says it will use enforcement discretion for otherwise lawful NAC supplements.
04How much NAC is in supplements?
Amounts vary a lot. RIVM found NAC supplements on the Dutch market recommending 200 to 2,000 mg a day, usually as 1 to 3 capsules.
Building on a 2015 assessment by the Norwegian Scientific Committee for Food Safety (VKM), RIVM set these reference values, which it considers not to lead to health risks:
| Adults (over 18) | 1,200 mg a day |
|---|---|
| 11 to 18 years | 770 mg a day |
| 6 to 11 years | 430 mg a day |
| 2 to 6 years | 250 mg a day |
| Under 2 years | Not considered safe |
RIVM noted that stomach and gut complaints can’t be ruled out above these amounts. Always follow the label and do not exceed the stated dose.
05Side effects and who should be careful
Side effects reported for acetylcysteine taken by mouth include an unpleasant taste, nausea, vomiting and diarrhoea, mostly from medicine use. RIVM, drawing on Dutch medicines information, highlights groups who should take extra care:
- Asthma or COPD: caution, because of a possible risk of the airways tightening (bronchospasm).
- A current or past stomach ulcer: caution, because NAC may irritate the lining of the gut.
- Histamine intolerance, severe liver or kidney disease, or cystinuria: speak to a doctor first.
- Pregnancy or breastfeeding: ask a healthcare professional before taking any supplement.
- Children: not for under-2s, and most NAC supplements are intended for adults.
06NAC and medicines
RIVM lists several possible interactions. Tell your GP or pharmacist about any supplement you take, and ask before combining NAC with:
| Nitroglycerin or isosorbide | May increase their effect, causing low blood pressure and headache; combining is strongly advised against |
|---|---|
| Carbamazepine | NAC may lower blood levels of this epilepsy medicine |
| Antibiotics | As a precaution, take them 2 hours before or after NAC |
| Activated charcoal | Each may weaken the other |
| Blood thinners | A theoretical increase in bleeding risk |
| Acetylcysteine medicines | Not recommended together, as total intake may exceed the reference value |
07How to read a NAC label
- Check the amount per daily dose, not per capsule, and add up NAC from all the products you take.
- Look for the “food supplement” name and the required warnings, such as not exceeding the recommended daily dose.
- Be wary of health promises. No health claims are authorised for NAC in Great Britain.
For more context, read our guides to H. pylori in the UK, gut health and diet and Pylopass™ explained.
08Frequently asked questions
What is NAC?
NAC (N-acetyl cysteine) is the N-acetyl form of the amino acid L-cysteine, which the body converts into L-cysteine. It is sold as a food supplement ingredient, and the same molecule, called acetylcysteine, is the active substance in some licensed medicines.
Is NAC legal in UK food supplements?
NAC is sold in UK food supplements, but its position sits close to the food–medicine borderline because acetylcysteine is also a licensed medicine. The MHRA decides whether a product counts as a medicine. No health claims are authorised for NAC in Great Britain.
How much NAC is in a typical supplement?
It varies. A 2024 Dutch government assessment found supplements recommending 200 to 2,000 mg a day, and set 1,200 mg a day as a reference value for adults, with lower values for children and none for under-2s. Always follow the label and do not exceed the stated dose.
What are the side effects of NAC?
Reported side effects of acetylcysteine taken by mouth include an unpleasant taste, nausea, vomiting and diarrhoea. People with asthma, a stomach ulcer, histamine intolerance or severe liver or kidney disease should speak to a doctor or pharmacist first.
Can I take NAC with my medicines?
Ask your GP or pharmacist first. Possible interactions include nitroglycerin or isosorbide, carbamazepine, activated charcoal, blood thinners and some antibiotics. Taking a NAC supplement alongside an acetylcysteine medicine is not recommended.
Is NAC the same as acetylcysteine medicine?
It is the same molecule. The difference is how it is regulated, the dose and the purpose. Acetylcysteine medicines are licensed and used under medical supervision; NAC in a food supplement is regulated as food and cannot carry health claims.
09About London Health Star
London Health Star is a UK company that makes food supplements. We list NAC by amount only and make no health claims about it.
Explore StarBiome Pylori, a food supplement with Pylopass™, NAC, chamomile and vitamin B12. Each daily dose of 2 vegan capsules contains:
- Pylopass™ (Lactobacillus reuteri DSM 17648, inactivated)200 mg
- N-acetyl cysteine (NAC)600 mg
- Chamomile extract37.5 mg
- Vitamin B12 (as methylcobalamin)25 µg (1000% NRV)
Vitamin B12 contributes to the reduction of tiredness and fatigue, to normal energy-yielding metabolism and to the normal function of the immune system.
Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle.
10Sources
Disclaimer: this article is for general information only. It is not medical or legal advice, and regulatory positions can change. Speak to your GP or pharmacist about your own situation.
- de Wit-Bos L, Zhao S. Risk assessment of food supplements containing N-acetylcysteine (NAC). RIVM letter report 2024-0086. National Institute for Public Health and the Environment (RIVM), 2024. https://www.rivm.nl/bibliotheek/rapporten/2024-0086.pdf
- Department of Health and Social Care (GOV.UK). Great Britain nutrition and health claims (NHC) register. https://www.gov.uk/government/publications/great-britain-nutrition-and-health-claims-nhc-register
- GOV.UK. Food supplements: guidance for businesses. https://www.gov.uk/government/publications/food-supplements-guidance-for-businesses/food-supplements
- MHRA (GOV.UK). Borderline products: how to tell if your product is a medicine. https://www.gov.uk/guidance/borderline-products-how-to-tell-if-your-product-is-a-medicine
- BNF (via NICE). Acetylcysteine. https://bnf.nice.org.uk/drugs/acetylcysteine/
- LiverTox (NCBI Bookshelf). Acetylcysteine. National Institute of Diabetes and Digestive and Kidney Diseases. https://www.ncbi.nlm.nih.gov/books/NBK548401/
- US Food and Drug Administration. Guidance for Industry: Policy Regarding N-acetyl-L-cysteine. August 2022. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-policy-regarding-n-acetyl-l-cysteine
- NHS. Anaphylaxis. https://www.nhs.uk/conditions/anaphylaxis/
