N-acetyl cysteine (NAC) is a white, crystalline powder derived from the amino acid cysteine, which serves as a precursor to glutathione, the body's most abundant antioxidant. Although NAC does not occur naturally in the diet, it is available as a dietary supplement. Cysteine, the amino acid from which NAC is derived, is found in many high-protein foods.
Many of the studies on NAC supplementation in bronchitis patients have used 600 mg per day. NAC appears to be well tolerated at this dosage level. Side effects such as hypotension and severe headaches have been reported in angina patients taking NAC with nitroglycerin. Although NAC appears to have very low toxicity, one study indicates that it may cause oxidative damage at dosages of 1200 mg or more per day.