What is NAC?
N-Acetyl Cysteine
It is an amino acid that has broad utility. It is prescribed for lung issues, but has many secondary uses including Covid prevention and treatment and concussion prevention and treatment. Too much can cause mold GI issues and potentially deficiency in some B-vitamins. Some say because of the b-vitamin issue that every second day is better than every day for it.
It is an acronym that the poster should have defined according to standard grammatical protocol. With no insult to impimp, I had to look it up too. It is N-Acetyl Cysteine, or NAC for short.