NAC is an amino acid (protein) that is an FDA approved drug that has been made an over the counter in the past year. I have been using NAC for my Long Covid and a recent bout of tendonitis. It was helpful. My wife recently had a cold/flu with a bad cough and I had her try NAC. It did help her cough and she would recommend it.
From WEB MD:
N-acetyl cysteine (NAC) comes from the amino acid L-cysteine. Amino acids are building blocks of proteins.
People commonly use N-acetyl cysteine for cough and other lung conditions. It is also used for flu, dry eye, and many other conditions, but there is no good scientific evidence to support many of these uses.