NAC is n-acetyl-cysteine and it is sold at GNC. I believe it is what they give APAP OD patients in the hospital, but I can't remember wht the brand name is(Mucomyst, or something like that). In layman's terms, it helps to clean up the toxic metabolites that your liver creates when it is trying to metabolize tylenol. The liver uses L-glutathione to do this. If you continually take tylenol, or take large doses, you run out of L-glutathione, and the toxic metabolites destroy your liver. NAC is the precursor to L-glutathione. If I have given misinformation, please forgive and correct me. I do know this, NAC seems to be a great supplement if you are taking tylenol on a regular basis, as is the case with all pain medications you can receive from online consultations. I told my Mom about NAC and she went digging through the internet, looking for studies and information to back up what I had told her. The information is definantly out there. Take NAC with Vitamin C and even a multivitamin, though, to keep all you amino acid levels in balance.