Cysteine is an amino acid that can be found in many proteins throughout the body. When used as a supplements, it is usually in the form of N-acetyl-L-cysteine (NAC). The body converts this to cysteine and then to glutathione, a powerful antioxidant.

Antioxidants fight free radicals, harmful compounds in the body that damage cell membranes and DNA. Free radicals occur naturally in the body, but environmental toxins (including ultraviolet light, radiation, cigarette smoking, and air pollution) can increase the number of these damaging particles. Free radicals are believed to 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 appears 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

Intravenous (IV) NAC is often given to people who have taken an overdose of acetaminophen (Tylenol), to prevent or reduce liver and kidney damage. Acetaminophen poisoning can also occur at lower doses if someone drinks alcohol or takes certain medications that may damage the liver on a regular basis.

Angina

In clinical studies of people with ongoing chest pain, NAC, in combination with nitroglycerin (a drug that opens up blood vessels and improves blood flow), has been more effective than either NAC or nitroglycerin alone in reducing subsequent 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 improve symptoms associated with chronic bronchitis, leading to fewer flare ups. However, one large study failed to find any reduction in flare ups among people with chronic bronchitis. In another study of people with moderate to severe COPD, taking NAC decreased the number of flare ups about 40% when used with other therapies.

Influenza

In one six-month study, people who took 600 mg of NAC twice daily had fewer flu symptoms than those who took placebo.

Acute Respiratory Distress Syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) develops after an injury or trauma to the lungs and is life-threatening. Although not all studies agree, some research (laboratory and human) 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 cysteine) to people with ARDS helped reduce the severity of their condition. But it did not reduce the number of overall deaths compared to placebo.

HIV/AIDS

Some researchers have investigated whether cysteine can help improve levels of glutathione in people with HIV or AIDS. In one well-designed clinical study of people with HIV, those 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. Similarly, in a smaller-scale clinical study where HIV positive patients took NAC, the supplement did increase glutathione levels while a placebo did not. Other clinical studies, however, have shown negative results using NAC for those with HIV. 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 evidence is limited:

• Reducing symptoms associated with Sjögren's syndrome (an autoimmune disorder characterized by dry mouth and dry eyes)
• Reducing symptoms of asthma, cystic fibrosis, and emphysema
• Preventing colon cancer
• Preventing cataracts and macular degeneration
• Helping increase HDL "good" cholesterol
• Helping increase fertility when taken along with fertility drugs in people with polycystic ovary disease
• Helping improve outcome in children with advanced cerebral adrenoleukodystrophy, a type of leukemia
• Helping treat cocaine addiction, schizophrenia, and gambling addiction
• Reducing lung cancer risk among smokers

More studies are needed.

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

Pediatric

A child should not take NAC unless under the supervision of a doctor.

Adult

Recommended adult doses of NAC vary depending on the health condition being treated.

NAC is given either intravenously or orally in the hospital to treat acetaminophen (Tylenol) poisoning in both children and adults. Acetaminophen poisoning is a medical emergency, and treatment must be started within 8 hours of an overdose.

• 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. Acute respiratory distress syndrome is a medical emergency and is treated in the hospital with intravenous NAC as part of a comprehensive treatment regimen.
• For antioxidant protection/general health: Take 500 mg daily to start. The dosage may be increased, with your doctor's supervision. Someone with HIV/AIDS may be put on a dose as high as 4,000 mg daily. 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, dietary supplements should be taken only under the supervision of a knowledgeable health care provider.

Some forms of cysteine are toxic and should be avoided. These include 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.

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

Oral NAC 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 (along with three other amino acids) is lost in the urine, should not take cysteine supplements.

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.

Angiotensin-converting enzyme (ACE) inhibitors (blood pressure medication) -- NAC may increase the blood pressure-lowering effects of ACE inhibitors. Examples of ACE inhibitors include:

• Captopril (Capoten)
• Enlapril (Vasotec)
• Lisinopril (Zestril, Prinivil)
• Fosinopril (Monopril)

Immunosuppressive medications -- Treatment with NAC may stregthen 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.

Cisplatin and doxorubicin -- Laboratory and animal studies have suggested that NAC may reduce the toxic effects associated with cisplatin (Platinol) and doxorubicin (Ardiamycin). These medications are used to treat a variety of cancers. However, scientific studies are needed to see if this is true in humans.

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 increase 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 -- Topical applications of NAC may strengthen the effect of oxiconazole (Oxistat), an antifungal medication used for athlete's foot.

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