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Complementary and Alternative Medicine


Also listed as: Acetylcysteine; L-cysteine; N-acetylcysteine; NAC
Table of Contents > Supplements > Cysteine     Print

Dietary Sources
Available Forms
How to Take It
Possible Interactions
Supporting Research


Cysteine is an amino acid, a building block of proteins that are used throughout the body. When taken as a supplement, it is usually in the form of N-acetyl-L-cysteine (NAC). The body makes this into cysteine and then into glutathione, a powerful antioxidant.

Antioxidants fight free radicals, harmful compounds in the body that damage cell membranes and DNA. Researchers think free radicals play a role in aging as well as the development of a number of health problems, including heart disease and cancer.

NAC can help prevent side effects caused by drug reactions and toxic chemicals, and helps break down mucus in the body. It seems to have benefits in treating some respiratory conditions, such as bronchitis and COPD.


N-acetyl cysteine (NAC) may be used in preventing or treating the following conditions:

Acetaminophen poisoning

Doctors often give intravenous (IV) NAC to people who have taken an overdose of acetaminophen (Tylenol), to help prevent or reduce liver and kidney damage. Acetaminophen poisoning can also happen at lower doses if someone drinks alcohol or takes medications that may damage the liver on a regular basis. Acetaminophen poisoning is a medical emergency and can happen because of an accidental overdose. If you think someone has taken an overdose of acetaminophen, take them to the hospital.


In clinical studies of people with ongoing chest pain, taking NAC along with nitroglycerin, a drug that opens up blood vessels and improves blood flow, has been more effective than taking either one alone in reducing chest pain, heart attack, and the risk of death. However, the combination can also cause a severe headache. You should not try to treat chest pain on your own. Always see a doctor.

Chronic bronchitis and Chronic obstructive pulmonary disease (COPD)

A review of clinical studies found that NAC may help relieve symptoms of chronic bronchitis, leading to fewer flare ups. But not all studies agree. One large and well-designed study didn’t find any reduction in flare ups. In another study of people with moderate-to-severe COPD, taking NAC lowered the number of flare ups about 40% when used with other therapies.


In one 6-month study, people who took 600 mg of NAC two times a day had fewer flu symptoms than those who took placebo.

Acute Respiratory Distress Syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) happens after an injury to the lungs and is life-threatening. Although not all studies agree, some research -- laboratory and in people -- suggests that intravenous NAC may boost levels of glutathione and help prevent and/or treat lung damage caused by ARDS. However, results of other studies have been conflicting. In one study, giving NAC or procysteine, a synthetic amino acid, to people with ARDS helped reduce the severity of their condition. But it did not reduce the number of overall deaths compared to placebo. ARDS is a medical emergency -- you should not try to treat it at home.


Some researchers have looked at whether cysteine can help boost levels of glutathione in people with HIV or AIDS. In one well-designed clinical study, people with HIV who took daily supplements including the amino acid glutamine (40 grams per day), vitamin C (800 mg), vitamin E (500 IU), beta-carotene (27,000 IU), selenium (280 mcg), and N-acetylcysteine (2400 mg) gained more weight after 12 weeks than those who took placebo. In a smaller-scale clinical study where HIV-positive patients took NAC, the supplement did boost glutathione levels compared to placebo. But other studies have had negative results. More research is needed to see whether NAC has any benefit for people with HIV.

Other Uses

NAC has also been proposed for the following conditions, although there is not much evidence:

  • Reducing symptoms associated with Sjögren syndrome, an autoimmune disorder that causes dry mouth and dry eyes
  • Reducing symptoms of asthma, cystic fibrosis, and emphysema
  • Preventing colon cancer
  • Helping increase fertility when taken along with fertility drugs in people with polycystic ovary disease
  • Helping treat schizophrenia
  • Reducing lung cancer risk among smokers

More studies are needed.

Dietary Sources

Your body makes cysteine from the essential amino acid methionine. Cysteine is also found in most high-protein foods, including ricotta, cottage cheese, yogurt, pork, sausage meat, chicken, turkey, duck, lunch meat, wheat germ, granola, and oat flakes. NAC is not found in food.

Available Forms

  • NAC aerosol spray (prescription)
  • NAC liquid solution (prescription)
  • NAC topical solution
  • L-cysteine powder
  • Cysteine/NAC tablets or capsules

How to Take It


Don’t give NAC to a child except under a doctor’s supervision.


Recommended adult doses of NAC vary depending on the health condition being treated. Below are some examples for illustration purposes only. Talk to your doctor to find the safest, most effective dose for your condition.

  • NAC is given either by IV or mouth in the hospital to treat acetaminophen (Tylenol) poisoning in both children and adults. Acetaminophen poisoning is a medical emergency, and should receive immediate medical care.
  • For adults 18 years and older with respiratory illness: 200 mg, 2 times daily, for chronic bronchitis. For COPD, 600 mg, 2 times daily, has been used.
  • For antioxidant protection/general health: Take 500 mg daily to start. The dosage may be increased, with your doctor's supervision.

Adding a multivitamin will ensure that you are getting the B vitamins you need when taking NAC.


Because of the potential for side effects and interactions with medications, take dietary supplements should only under the supervision of a knowledgeable health care provider.

Avoid some forms of cysteine, as they are toxic: D-cysteine, D-cystine, and 5-methyl cysteine.

NAC may raise levels of homocysteine, an amino acid that is associated with heart disease. Be sure to have your health care provider check your homocysteine level if you are taking NAC.

Very high doses (more than 7 grams) of cysteine may be toxic to human cells and may even lead to death.

Taking NAC by mouth may cause nausea, vomiting, and diarrhea.

Intravenous administration of NAC to treat acetaminophen poisoning may cause severe allergic reactions, including angioedema, swelling of the soft tissue just beneath the skin including the face, lips, and around the eyes; or anaphylaxis, a life-threatening allergy.

People with cystinuria, a kidney condition in which too much cysteine is lost in the urine, should not take cysteine supplements.

When inhaled into the lungs, NAC may cause tightness in the chest, numbness of the mouth, runny nose, and drowsiness. It may make asthma symptoms worse. People with asthma who are taking NAC should be watched closely by their doctor.

Possible Interactions

If you are currently being treated with any of the following medications, you should not use cysteine supplements without first talking to your health care provider.

Medications that suppress the immune system -- Treatment with NAC may strengthen the effects of some medications that suppress the immune system, such as azathioprine (Imuran), cyclophosphamide (Cytoxan), or prednisone (Deltasone). Do not take NAC with these medications without talking to your doctor first.

Nitroglycerin and isosorbide -- NAC may strengthen the effect of nitroglycerin and isosorbide (Isordil), two medications commonly used to treat chest pain. But this combination may also raise the risk of side effects, such as severe headaches, and may lead to abnormally low blood pressure. Do not take NAC with these medications unless your doctor tells you to do so.

Oxiconazole -- Using NAC on the skin strengthens the effect of oxiconazole (Oxistat), an antifungal medication used for athlete's foot.

Activated charcoal -- may make NAC les effective.

Supporting Research

Adair JC, Knoefel JE, Morgan N. Controlled trial of N-acetylcysteine for patients with probable Alzheimer's disease. Neurology. 2001;57(8):1515-1517.

Ames BN. Micronutrient deficiencies: A major cause of DNA damage. Ann NY Acad Sci. 2000;889:87-106.

Badawy A, State O, Abdelgawad S. N-Acetyl cysteine and clomiphene citrate for induction of ovulation in polycystic ovary syndrome: a cross-over trial. Acta Obstet Gynecol Scand. 2007;86(2):218-22.

Cai J, Nelson KC, Wu M, Sternberg P Jr, Jones DP. Oxidative damage and protection of the RPE. Prog Retin Eye Res. 2000;19(2):205-221.

Chevez-Barrios P, Wiseman AL, Rojas E, Ou CN, Lieberman MW. Cataract development in gamma-glutamyl transpeptidase deficient mice. Exp Eye Res. 2000;71(6):575-582.

De Rosa SC, Zaretsky MD, Dubs JG, Roederer M, Anderson M, Green A, et al. N-acetylcysteine replenishes glutathione in HIV infection. Eur J Clin Invest. 2000;30:915-929.

El-Hamamsy I, Stevens LM, Carrier M, et al. Effect of intravenous N-acetylcysteine on outcomes after coronary artery bypass surgery: a randomized, double-blind, placebo-controlled clinical trial.J Thorac Cardiovasc Surg. 2007 Jan;133(1):7-12.

Ghabril M, Chalasani N, Björnsson E. Drug-induced liver injury: a clinical update. Curr Opin Gastroenterol. 2010 May;26(3):222-6. Review.

Goodman MT, McDuffie K, Hernandez B, Wilkens LR, Selhub J. Case-control study of plasma folate, homocysteine, vitamin B12, and cysteine as markers of cervical dysplasia. Cancer. 2000;89:376-382.

Kozer E, Koren G. Management of paracetamol overdose: current controversies. [Review]. Drug Saf. 2001;24(7):503-512.

Mardikian PN, LaRowe SD, Hedden S, Kalivas PW, Malcolm RJ. An open-label trial of N-acetylcysteine for the treatment of cocaine dependence: a pilot study. Prog Neuropsychopharmacol Biol Psychiatry. 2007;31(2):389-94.

Mazer M, Perrone J. Acetaminophen-induced nephrotoxicity: pathophysiology, clinical manifestations, and management. J Med Toxicol. 2008 Mar;4(1):2-6. Review.

Micke P, Beeh KM, Schlaak JF, Buhl R. Oral supplementation with whey proteins increases plasma glutathione levels of HIV-infected patients. Eur J Clin Invest. 2001;31(2):171-178.

Muller F, Svardal AM, Nordoy I, Berge RK, Aukrust P, Froland SS. Virological and immunological effects of antioxidant treatment in patients with HIV infection. Eur J Clin Invest. 2000;30(10):905-914.

Novelli EL, Santos PP, Assalin HB, Souza G, Rocha K, Ebaid GX, et al. N-acetylcysteine in high-sucrose diet-induced obesity: energy expenditure and metabolic shifting for cardiac health. Pharmacol Res. 2009 Jan;59(1):74-9.

Ozkilic AC, Cengiz M, Ozaydin A, Cobanoglu A, Kanigur G. The role of N-acetylcysteine treatment on anti-oxidative status in patients with type II diabetes mellitus. J Basic Clin Physiol Pharmacol. 2006;17(4):245-54.

Silva LA, Silveira PC, Pinho CA, Tuon T, Dal Pizzol F, Pinho RA. N-acetylcysteine supplementation and oxidative damage and inflammatory response after eccentric exercise. Int J Sport Nutr Exerc Metab. 2008 Aug;18(4):379-88.

Stey C, Steurer J, Bachmann S, Medici TC, Tramer MR. The effect of oral N-acetylcysteine in chronic bronchitis: a quantitative systematic review. Eur Respir J. 2000 Aug;16(2):253-262.

Sutherland ER, Crapo JD, Bowler RP. N-acetylcysteine and exacerbations of chronic obstructive pulmonary disease. COPD. 2006;3(4):195-202.

Tolar J, Orchard PJ, Bjoraker KJ, Ziegler RS, Shapiro EG, Charnas L. N-acetyl-L-cysteine improves outcome of advanced cerebral adrenoleukodystrophy. Bone Marrow Transplant. 2007;39(4):211-5.

Review Date: 5/1/2011
Reviewed By: Steven D. Ehrlich, NMD, Solutions Acupuncture, a private practice specializing in complementary and alternative medicine, Phoenix, AZ. Review provided by VeriMed Healthcare Network.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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