NAC is a form of the amino acid cysteine that helps the body produce a strong antioxidant called glutathione. It also helps reduce respiratory distress symptoms.
I can't find it at the moment, but I've also read that NAC helps to remove the spike proteins both from covid and from the vax shots.
My Dr., although she was against using ivermectin, was a strong proponent of NAC, and said that she took it daily for the first covid year, despite seeing sick patients all the time, and claims that it was the NAC that kept her from getting it.
Here's a study about the use of NAC with covid. It's basically an amino acid that has typically been widely available. However, early on in covid, as people were buying it up, the FDA changed it to be considered a medication, and not a supplement, which made it harder to come by. You can get it pretty easily now though, just not on Amazon.
NAC study
It's safe, and really the only thing you need to do is drink plenty of water when you're taking it, so IMO, it's worth a shot. I took it throughout my bout with covid and never fully lost my smell, though it was probably like 90% gone, but it came back within a week.