NAC N Acetyl L Cysteine Allergy Research Group As I mention in my book, N acetyl cysteine (NAC) is not found in food sources, but in the body, it converts into cysteine. NAC acts as a precursor to glutathione, a powerful As a precursor to make glutathione, NAC has been used for decades both as an acute medication and as an over the counter supplement. Recently, the line between medication and supplement has been N AcetylL Cysteine 600 mg Capsules (NAC) Natural Factors USA NAC Supplement for Glutathione Boost and Liver Support