N-acetylcysteine (NAC) supplements: evidence and safety

N-acetylcysteine is a modified form of the amino acid cysteine. Cysteine is a building block for glutathione, one of the body's main antioxidants, which is why NAC is described as a glutathione precursor. This chemistry explains both its established medical role and the theories behind newer uses.

A review of psychiatric applications explains that NAC may also influence glutamate signalling in the brain, oxidative stress and inflammation, three processes thought to be involved in several mental health conditions. These are proposed mechanisms rather than proven reasons for treating anyone.

A safety review of NAC at higher doses in chronic respiratory disease notes that the drug has been used worldwide for over fifty years. It is the antidote for paracetamol poisoning, given intravenously in hospital to prevent liver necrosis, and has been used in contrast-induced kidney injury. In respiratory medicine it is used as a mucolytic, helping to thin secretions, in conditions such as chronic obstructive pulmonary disease, bronchiectasis, chronic bronchitis and cystic fibrosis.

The same review points out that the maximum licensed dose for chronic use in respiratory disease is lower than some doses tested in trials, a reminder that medicinal use is set by licences and clinical oversight. Anyone prescribed acetylcysteine by a doctor should follow that prescription rather than adjusting it with supplements.

A narrative review in BioMed Research International summarised systematic reviews and trials. It reported good evidence, from a small number of well-designed trials, for NAC as an add-on treatment to reduce total and negative symptoms of schizophrenia. For substance use disorders it found NAC seemed to reduce craving, particularly for cocaine and cannabis among young people, and might help prevent relapse in people already abstinent.

For obsessive-compulsive and related disorders the results were mixed, and for bipolar disorder and depression the evidence remained unclear, with conflicting findings between reviews. The authors noted that most studies used NAC alongside existing medicines, so little is known about NAC on its own, and called for larger, better-designed trials. A scoping review in children and adolescents likewise concluded that the evidence base is still limited.

• NAC is not a recognised first-line treatment for any of these conditions in UK practice.

• Standard care for conditions such as OCD, schizophrenia or depression is assessed through the GP and mental health services.

• If you are already taking psychiatric medicines, tell your prescriber before adding any supplement.

In the respiratory safety review of forty-one studies, adverse effects were usually mild and mostly gastrointestinal, such as stomach upset and diarrhoea, and generally no more common than in control groups, even at doses above the standard licensed level. The psychiatric review described oral NAC as safe and well tolerated in the trials it covered.

Trials are short and carefully selected, so they cannot rule out problems in everyday use, in pregnancy, or in combination with other medicines. The FSA advises checking with a GP or pharmacist before taking a food supplement if you are pregnant, breastfeeding, have a medical condition or take prescribed medicines, and stopping and seeking advice if you feel unwell.

Acetylcysteine is a licensed medicine for specific uses, and a food supplement is a different legal category. The FSA explains that food supplements are not medicinal products and should not be sold with claims to prevent, treat or cure a medical condition; medicines make such claims under a licence. I did not find a UK regulator page stating how every NAC capsule is classified, so the safe approach is to read the label, check who the seller is and ask a pharmacist.

Because overdose of paracetamol-containing products can be silent at first, anyone who has taken too much, intentionally or by accident, should go to A&E or call NHS 111 or 999 straight away. NAC is given in hospital as part of that treatment, not self-administered at home.