Cysteine is an amino acid that can be found in many proteins throughout the body. N-acetyl-L-cysteine (NAC), a modified form of cysteine, helps break down mucus and detoxify harmful substances in the body. Both cysteine and NAC have been shown to increase levels of the antioxidant glutathione.

Antioxidants are substances that scavenge free radicals, damaging compounds in the body that alter cell membranes, tamper with DNA, and even cause cell death. Free radicals occur naturally in the body, but environmental toxins (including ultraviolet light, radiation, cigarette smoking, and air pollution) can also increase the number of these damaging particles. Free radicals are believed to contribute to the aging process as well as the development of a number of health problems, including heart disease and cancer. Antioxidants such as glutathione can neutralize free radicals and may reduce or even help prevent some of the damage they cause.

N-acetyl cysteine (NAC) offers a variety of potential therapeutic uses, particularly in the prevention or in the treatment of the following conditions:

Acetaminophen poisoning

Health care providers commonly administer oral or intravenous (IV) NAC to prevent or reduce liver and kidney damage associated with overdoses of acetaminophen (Tylenol), an over-the-counter (OTC) medication commonly used for pain or headache. Acetaminophen poisoning can occur at lower doses of the drug if someone drinks alcohol or takes certain medications that may damage the liver on a regular basis.

Heart disease

In clinical studies of people having a heart attack or those 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, individuals who receive both NAC and nitroglycerin may experience a severe headache. These results are promising, but further human studies are needed to confirm the safety and effectiveness of NAC for heart disease.

A large clinical study found that administering NAC after coronary bypass surgery did not improve outcomes.

Respiratory illness

A review of clinical studies also found that NAC may help dissolve mucus and improve symptoms associated with chronic bronchitis, asthma, cystic fibrosis and emphysema. Chronic smokers also may benefit from NAC supplementation. Clinical studies on large groups of people have found that NAC appears to have cancer prevention properties in people who are at risk for lung cancer.

Free radical damage is believed to contribute to the development and progression of acute respiratory distress syndrome (ARDS), a condition characterized by the rapid and progressive malfunction of the lungs. Although not all clinical studies agree, some research (laboratory and human) suggests that intravenous NAC may boost levels of glutathione and subsequently prevent and/or treat lung damage caused by ARDS. However, results of other studies have been conflicting. Further investigation is needed.

HIV/AIDS

Human immunodeficiency virus (HIV) infection is considered to be a condition of excessive oxidative stress (caused by free radicals) where the antioxidant glutathione is depleted significantly. Therefore, it is believed that supplementation with cysteine may help strengthen the immune system in those with HIV and diminish the bodily damage associated with this infection.

In one well-designed clinical study of people with HIV, those who took a daily supplement regimen 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 significantly more weight after 12 weeks than those who took placebo. Similarly, in a smaller-scale clinical study using NAC in HIV positive patients, the supplement did increase glutathione levels while a placebo did not.

These studies and others support the theory that NAC may prove to be a useful addition to conventional medical care for those with HIV. Other clinical studies, however, have shown negative results using NAC for those with HIV. Therefore, more research is needed before conclusions can be drawn about NAC supplements for this infection.

Others

Low levels of cysteine may be linked to an increased risk of cervical dysplasia (changes to the opening of the uterus that are precancerous or cancerous). Preliminary evidence also suggests that NAC supplements:

• May improve symptoms associated with Sjögren's syndrome (an autoimmune disorder characterized by dry mouth and dry eyes).
• Help enhance cognitive functioning in some individuals with Alzheimer's disease
• Prevent development of cataracts and macular degeneration
• Slow down motor impairment in amyotrophic lateral sclerosis (ALS, often called Lou Gehrig's disease which is a progressive loss of control of voluntary muscles due to destruction of nerve cells in the brain and spinal cord)
• Help treat hepatitis C when combined with standard medical treatment
• Help increase HDL cholesterol (the good kind of cholesterol)
• Help increase fertility along with fertility drugs in people with polycystic ovary disease
• Help improve outcome in children with advanced cerebral adrenoleukodystrophy, a type of leukemia
• May be useful in treating cocaine addiction. Twenty-three cocaine addicts completely or significantly reduced their use of cocaine during treatment with NAC in a small clinical study.

Further studies are needed to confirm these early findings.

The body synthesizes 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

NAC is administered 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.

Pediatric

There are no specific pediatric recommendations for the dietary supplement cysteine. If laboratory tests reveal that the child has an amino acid imbalance that requires treatment, a health care provider may recommend a complete amino acid supplement that contains cysteine.

Adult

Recommended adult doses of NAC vary depending on the health condition being treated. The following list provides guidelines for the most common uses.

• For adults 18 years and older with respiratory illness: Take 200 mg, 2 times daily, for chronic bronchitis. Acute respiratory distress syndrome is a medical emergency and is treated in the intensive care unit 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 guidance by a health care provider. 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 believed to play a role in the development of 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 should be avoided because it 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, for example, acetaminophen poisoning) may cause severe allergic reactions, even angioedema (significant swelling of the soft tissue just beneath the skin including the face, lips, and around the eyes) or anaphylaxis (a life-threatening allergy that leads to inability to breath). There has been one case report of seizures from intravenous administration of NAC to a young girl for paracetamol intoxication.

Cysteine supplements should not be taken by individuals with cystinuria, a kidney condition in which excessive amounts of cysteine (along with three other amino acids) are lost in the urine.

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.

Blood pressure medications, angiotensin-converting enzyme (ACE) inhibitors -- NAC may enhance the blood pressure-lowering effects of ACE inhibitors, medications commonly used to treat high blood pressure. Examples of ACE inhibitors include benazepril (Lotensin), captopril (Capoten), enalapril (Vasotec), and lisinopril (Prinivil).

Immunosuppressive medications -- Treatment with NAC may enhance the effectiveness of immunosuppressive medications such as azathioprine (Imuran), cyclophosphamide (Cytoxan), or prednisone (Deltasone). More research in this area is needed.

Cisplatin and doxorubicin -- Laboratory and animal studies have suggested that NAC may reduce the toxic effects associated with both cisplatin (Platinol) and doxorubicin (Ardiamycin), medications used to treat a variety of cancers. However, scientific studies are needed to see if these effects apply to people.

Nitroglycerin and isosorbide -- Although NAC may enhance the effectiveness of nitroglycerin and isosorbide (Isordil), two medications commonly used to treat chest pain. This combination may also increase the risk of side effects such as severe headaches and may lead to abnormally low blood pressure.

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

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