N-Acetylcysteine (NAC) Supplements: Health Benefits, Uses, and Side Effects (2024)

N-acetylcysteine (NAC) supplements are the synthetic form of the amino acid cysteine. Cysteine is a nonessential (or semi-essential) amino acid. This means your body produces some of what you need and you also get some from food.

Claims abound about the benefits of NAC supplements for certain health conditions, like respiratory diseases, heart disease, and mental health disorders. Some are backed by preliminary research while others aren't.

However, NAC has the unusual distinction of being approved by the Food and Drug Administration (FDA) for some uses.

This article explores the claims about NAC benefits and what the research shows. It also covers side effects, precautions, what to look for when buying it, and why some NAC supplements have been pulled off of store shelves.

Health Benefits of N-Acetylcysteine

Little or no evidence supports most purported NAC benefits. Research to date has been preliminary. Some of those findings are promising, though, and warrant further research.

Some research suggests NAC supplements can increase the amount of glutathione in your body. Glutathione is a powerful antioxidant. To create it, NAC bonds with two other amino acids—glutamine and glycine.

Glutathione plays essential roles in the body, including:

• Regulating cellular activities
• Keeping the immune system in check
• Neutralizing free radicals that damage cells and tissues

Integrative medicine practitioners suggest the glutathione increase might help prevent and manage some health conditions. These include:

• Acetaminophen poisoning
• Lung disease, including chronic obstructive pulmonary disease (COPD)
• COVID-19 and other viral illnesses
• Heart disease
• Male infertility
• Polycystic ovary syndrome
• Mental health disorders
• Neurological disease
• Inflammatory bowel disease
• Obstructive sleep apnea

You should not use NAC supplements for the sole treatment or prevention of any of the above conditions. Before using a supplement, talk to your healthcare provider. They can determine whether it's safe for you based on your medical history and what else you're taking.

Acetaminophen (Tylenol) Poisoning

The FDA doesn't often approve supplements for medicinal use, but NAC has the agency's stamp of approval for treating Tylenol (acetaminophen) poisoning. Acetaminophen is safe at low doses, but overdose can cause liver toxicity and failure, which can be fatal.

In overdose cases, healthcare providers can use intravenous (IV) infusions of NAC to prevent liver damage and other symptoms. (IV means it's delivered directly into a vein via a needle.) The NAC is usually given in three infusions over the course of 21 hours.

Lung Disease

Some studies have looked into the NAC benefits in people with lung diseases such as:

• Chronic bronchitis
• COPD
• Cystic fibrosis

A 2015 meta-analysis published in European Respiratory Review evaluated 13 studies involving 4,155 people with COPD. It concluded that 1,200 milligrams (mg) of NAC per day reduced the incidence and severity of flares compared to a placebo.

The FDA has approved inhaled NAC as an add-on treatment for helping break up mucus in lung conditions.

COVID-19 and Other Viral Illnesses

Due to the antioxidant and anti-inflammatory characteristics of NAC, it has been used in clinical practice for people hospitalized with COVID-19.

A 2021 study published in Infectious Diseases looked at the effects of NAC supplementation. Researchers said taking 600 mg NAC orally (by mouth) twice daily for 14 days led to:

• Reduced disease progression
• Reduced need for intubation (a breathing tube)
• Fewer deaths

But the study authors concluded that larger, well-designed trials need to be run to determine how safe and effective this treatment is. Two clinical trials quickly got underway.

Some evidence also supports using NAC to help treat:

• Influenza (the flu)
  
• Respiratory syncytial virus (RSV)
• Tuberculosis
• Heliobacter pylori (H. pylori) infection

Heart Disease

Proponents of NAC say it may help lower the risk of heart disease by reducing oxidative stress on the heart. Oxidative stress occurs when an imbalance of free radicals (unstable molecules) damages the body's cells and tissues.

In research, daily use of NAC reduced hypertension (high blood pressure), which is a significant contributor to atherosclerosis (plaque build-up in the arteries).

A 2015 study in the American Journal of Clinical Nutrition says people taking NAC supplements for four weeks had a significant drop in systolic (top number) and diastolic (bottom number) blood pressure. These drops occurred regardless of their weight, cholesterol levels, or whether they smoked.

The same study also suggested NAC may reduce your blood's hom*ocysteine levels. High levels of hom*ocysteine can increase your risk of heart disease. However, the long-term effects of NAC on blood pressure are unknown.

This study was too small to draw firm conclusions but provided promising evidence that may encourage further study.

Male Infertility

Research has looked at whether NAC may further improve fertility in biological males with varicoceles, enlarged veins in the scrotum and testicl*s. Varicoceles is a leading cause of male infertility.

According to a 2016 study, people who underwent surgery to treat varicoceles had higher conception rates if they took NAC before and after surgery. Again, though, this was a small, preliminary study.

Polycystic Ovary Syndrome

NAC may help alleviate symptoms of polycystic ovary syndrome (PCOS), according to a 2015 systematic review.

The review evaluated eight studies with a total of 910 people with PCOS. Investigators found NAC improved ovulation and pregnancy rates compared to a placebo. However, the improvement was not as great as it was with metformin (a prescription drug used to treat PCOS).

Researchers also reported NAC did not improve other common symptoms of PCOS, such as menstrual irregularities or weight gain.

Mental Health Disorders

NAC may play a role in treating mental health conditions related to inflammation and dysregulation of the neurotransmitter (brain chemical) glutamate.

Preliminary results have mostly been inconclusive, but some research suggests NAC supplements may be an effective add-on treatment for:

• Mood disorders, including major depressive disorder and bipolar disorder
• Obsessive-compulsive disorder (OCD)
• Schizophrenia
• Substance abuse disorders, especially involving cocaine and cannabis (marijuana)

NAC supplements shouldn't be used alone for treating any mental health disorders. You should look at it as an additional treatment, and only after clearing it with your healthcare provider.

Neurological Diseases

NAC may have a use as an add-on treatment for some neurological conditions, including:

• Parkinson's disease
• Alzheimer's disease/dementia
• Multiple sclerosis
• Peripheral and diabetic neuropathy
• Stroke rehabilitation

This is also believed to be due to its antioxidant and anti-inflammatory properties.

Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is an umbrella term for Crohn's disease (CD) and ulcerative colitis (UC). Oxidative stress is believed to be involved in these conditions, so antioxidant treatments have gotten a fair amount of attention.

In studies, NAC supplements helped prevent relapses in people who were tapering off of the drug prednisone, both with CD and UC.

Sleep Apnea

Sleep apnea causes you to stop breathing for short periods while you sleep. It's considered a cause of inflammation and a risk factor for high blood pressure, stroke, and heart disease.

Some small studies have suggested that NAC supplements may help improve nighttime breathing and reduce snoring in people with sleep apnea.

Cysteine Deficiency

Since cysteine is made in the body and found in high-protein foods, deficiency is rare.

However, if you're a vegetarian or vegan, you may be at risk of cysteine deficiency, especially if you don't eat a lot of cysteine-rich plants.

Side Effects

As with all supplements, side effects are possible when taking NAC. They're more common at high doses and different for different forms of the supplement.

Possible side effects from oral NAC supplements include:

• Nausea
• Vomiting
• Diarrhea
• Itching
• Skin redness

NAC supplements smell like rotten eggs, which may contribute to nausea and vomiting. Using forms that mask the taste, such as flavored tablets that dissolve in water, can help with that.

Inhaled NAC may cause:

• Runny nose
• Drowsiness
• Tightness in the chest
• Numbness of the mouth

If you have asthma, your healthcare provider should monitor you for reactions to inhaled NAC.

Allergies to NAC are uncommon but can occur. The risk is highest during NAC infusions. In rare cases, an infusion may cause anaphylaxis, a life-threatening allergic reaction.

Precautions

Although NAC is generally safe when used as directed, it's not safe for everyone or at all times of life.

• Pregnancy or breastfeeding: Don't take without approval from your healthcare provider.
• Children: A safe and effective oral dose of NAC hasn't been established. Don't give it to children unless directed to by a healthcare provider.
• People with bleeding disorders: NAC can slow blood clotting. Don't take it if you have a bleeding disorder such as hemophilia or Von Willebrand disease.
• People with kidney disease: Conflicting research means guidelines for NAC in kidney disease aren't established. Talk to your healthcare provider first before using NAC.
• People with asthma: Use caution with NAC unless done under medical supervision. NAC may cause bronchospasm when inhaled or taken orally.

Since NAC may affect blood tests and slow blood clotting, you may need to stop taking it 12 hours before a blood draw and for two weeks before elective surgeries.

Dosage of NAC

Most NAC supplements are sold in 500 mg dosages, although some are as high as 1,200 mg. Product suggested doses range from one to four doses per day for a total of 500 mg to 3,000 mg daily.

However, since the FDA does not regulate supplements, NAC doesn't have universal guidelines for its use. Use caution and work with a qualified healthcare provider when considering this (and any) supplement.

What Happens If I Take Too Much?

Oral NAC hasn't been tied to any cases of toxicity (overdose). However, taking too much NAC may make side effects more likely.

Accidental overdoses of IV NAC have caused deaths.

Interactions

NAC may intensify or interfere with the action of some medications, including:

• Angina medications: NAC may intensify the effects of nitrates for angina (heart pain from reduced blood flow). This may cause severe headaches.
• Blood thinners: NAC may further contribute to bleeding. You shouldn't take NAC without medical supervision.
• Antihypertensive medication: NAC may further lower blood pressure and lead to dangerous lows (hypotension).
• Activated charcoal: NAC supplementation may interfere with its intended action.

How to Store NAC

NAC supplements should be stored at room temperature in a cool, dry room. Discard any supplements that have expired, are discolored, or show signs of deterioration.

It is important to note that NAC may have an odor similar to sulfur, which is normal.

How to Buy

NAC is available over-the-counter (OTC) in several formulations, including:

• Tablets
• Capsules
• Softgels
• Effervescent tablets
• Powders

Are NAC Supplements Being Banned?

Because the FDA has approved NAC for some medical uses, it's considered a drug in the United States. In July 2020, the FDA issued a reminder for manufacturers that an approved drug can't be sold as a dietary supplement, putting the legality of NAC supplements in question.

Since the FDA's full safety review on NAC supplements is ongoing, the agency issued a statement in August 2022 declaring that it would turn a blind-eye to the sale of NAC-containing dietary supplements until further notice. In other words, NAC-containing dietary supplements are permitted for the time being, so long as the product does not make false claims about its medical benefits.

Summary

NAC is an FDA-approved drug as an intravenous treatment for acetaminophen poisoning and an inhaled treatment for breaking up mucus in some lung conditions.

NAC supplements have some preliminary evidence supporting their use for managing symptoms of viral illnesses, including COVID-19, heart disease, male infertility, PCOS, mental health and neurological disorders, inflammatory bowel disease, and sleep apnea.

Side effects and drug interactions are possible with NAC supplements. Always check with your healthcare provider before starting any medicinal product.

NAC is currently sold as a dietary supplement in many forms, but the FDA may put an end to that because it's legally classified as a drug.