N-acetylcysteine, or NAC for short, is a pharmaceutical drug and nutritional supplement derived from an amino acid known as L-cysteine. Doctors may recommend NAC to reduce the intensity of the common cold and related viral infectious diseases such as influenza. The Natural Medicines Comprehensive Database, a compendium made by an independent research group that evaluates nutritional supplements, lists NAC as "likely safe" and "possibly effective" against flu-like symptoms.
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NAC is primarily prescribed as a drug for the lungs and nasal passage to treat chronic obstructive pulmonary diseases such as bronchitis and influenza-like conditions. It works as a cough medicine because of its ability to break the bonds in mucus, which makes it easier to cough them up. NAC also offers significant protection to cells. It supports the body's antioxidant systems during stress, infections and inflammatory conditions by raising the levels of the antioxidant glutathione.
Researchers have not tested the efficacy of NAC against the common cold, but a study during the 1991 and 1992 flu season found that patients taking NAC were less likely to have clinical influenza — 29 percent vs. 51 percent in the placebo group — and the episodes were, on average, much less severe. The study was detailed in a 2009 review in "American Academy of Family Physicians." It also gave the evidence for the effectiveness of NAC against influenza a grade of "B." Influenza and the common cold are caused by different viruses, but they share many of the same symptoms, including coughs, a sore throat and fever, although influenza is more severe. Treatments that work for one do not always work for the other — for example, vitamin C and zinc only work for the cold — but its efficacy as a cough medicine appears to improve the probability that it has some utility against the common cold.
Although it is not backed up by scientific studies, some doctors report that 1,000 mg of NAC three times per day lessens the effects of sinus congestion, a condition in which air cavities around your nose, eyes and cheeks become inflamed, according to the University of Maryland Medical Center. Sinusitis is a common disease that may be caused by a variety of infections, including the common cold. NAC works directly on the mucus to treat sinusitis.
The evidence of NAC as a possible treatment for colds is mostly circumstantial and anecdotal, but NAC is also safe and inexpensive, costing only about $12 to $15 a month, and the mechanisms of treatment are well-defined and documented, says the journal "American Academy of Physicians." A 2010 Consumer Report survey found that only 9 percent of people plan to use NAC as a treatment during the winter season, but it may be effective for many patients in dosages of 600 to 1,200 mg a day with meals. Adverse effects are rare at dosages of 1,200 mg twice daily or less and only manifest as gastrointestinal discomfort. Interactions may occur in patients taking nitroglycerin. Consult your doctor before using NAC.