Cysteine is an amino acid found in many proteins. Its derivative, N-acetyl-L-cysteine (NAC), is commonly found in food and is synthesized by the body. NAC is also available as a supplement. It performs a few important functions, including breaking down mucus; producing glutathione, an important antioxidant; and protecting the body from acetaminophen toxicity.

NAC offers a variety of potential therapeutic uses, particularly in the prevention and/or treatment of the following conditions:

• Respiratory diseases. NAC may reduce the lung injury that occurs in adult respiratory distress syndrome (ARDS). NAC may also improve some of the symptoms associated with chronic bronchitis.
• Cardiovascular disease. NAC, in combination with nitroglycerin (a drug that dilates blood vessels), has been shown to significantly reduce the incidence of acute heart attacks in patients with angina pectoris (pain in the center of the chest induced by exercise). NAC has also been shown to reduce damage caused by heart attack and to cause an increase in HDL-cholesterol ("good cholesterol") levels.
• Acetaminophen poisoning. NAC is commonly used in the treatment of acetaminophen overdoses, reducing kidney damage if given within eight hours.
• Corneal damage in the eye. Animal studies indicate that NAC may help reduce damage to corneal cells, such as cataracts, that is related to smoking.
• NAC may also have a therapeutic benefit for people who are HIV-positive and for those with Sjogren's syndrome, which involves dryness of the eyes. Please refer to the HIV/AIDs monograph for more information. 

The primary dietary sources of NAC include the following:

• wheat germ
• granola
• oat flakes
• ricotta
• cottage cheese
• yogurt
• pork, sausage meat
• chicken, turkey, duck
• luncheon meat

NAC is available as a supplement in several forms. These include the following:

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

To treat acetaminophen poisoning in a hospital, for children or adults, the typical dosage of NAC is 140 mg/kg body weight, followed four hours later by 70 mg/kg every four hours for an additional 17 doses. Treatment must be started within eight hours of an acetaminophen overdose. Oral NAC is typically administered for 72 hours; intravenous NAC for 20 to 52 hours.

Pediatric

There is no specific dietary recommendation for cysteine. If laboratory tests reveal that the child has an amino acid imbalance that requires treatment, the healthcare 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.

• Bronchial disease: 200 mg two times per day
• To improve HDL cholesterol: 1,200 to 3,600 mg per day
• Antioxidant protection/general health: 500 mg per day to start. Individuals may increase the dose up to 3,000 to 4,000 mg per day as tolerated.

NAC and cysteine have several potential side effects. Taking high doses (over 7 g) of cysteine may be harmful and should be avoided. Oral NAC may cause nausea, vomiting, and diarrhea. Intravenously administered NAC to treat acetaminophen poisoning may cause a variety of potentially severe reactions. Intravenous NAC combined with nitroglycerin may cause hypotension (low blood pressure).

In addition, individuals with cystinuria (a urinary condition) should avoid, or limit, their intake of cysteine supplements. Toxic forms of cysteine that should not be used are D-cysteine, D-cystine, and 5-methyl cysteine.

NAC may enhance the blood pressure-lowering effects of angiotensin-converting enzyme (ACE) inhibitors, medications commonly used to treat high blood pressure. You should consult your healthcare provider before adding this supplement to your medication regimen.

Treatment with NAC may offer benefits in patients on immunosuppressive therapy with azathioprine, cyclophosphamide, prednisolone, or prednisone. Consult your healthcare provider to determine whether NAC supplements may be appropriate for you.

Laboratory studies have demonstrated that NAC may reduce toxicity associated with both cisplatin and doxorubicin, medications used to treat a variety of cancers. However, scientific studies are needed to confirm these effects in people.

Although the combination of NAC with nitroglycerin and isosorbide, medications commonly used to treat angina and chest pain, increases the effects of these medications, it may also increase the chance for unpleasant side effects, particularly headaches, associated with these medications. Consult your healthcare provider before using NAC if you are currently taking either nitroglycerin or isosorbide.

Topical applications of NAC may increase the effectiveness of oxiconazole, an antifungal medication. If you are currently taking this medication, you should consult your healthcare provider to determine whether NAC supplements may be appropriate for you.

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