FDA banned OTC NAC sales after it found out it might help with COVID [1]. Can't have that. And yeah, I know you're suggesting it for something else, but it's hard to get now.
The FDA didn't ban it: it just cannot be marketed as a health supplement. Mostly because it was originally approved as a drug back in the 1960s. It would be like selling Tylenol as a vitamin. A dumb technicality that'll likely be reversed.
There are many brands listed on iHerb (all with "dietary supplement" prominently listed in the product description), though most of the items are marked "out of stock", so it looks like many people have been stocking up.
Well, no. It appears that the FDA became aware of companies marketing N-acetyl-L-cysteine as a hangover remedy and started the process of taking action at that point. And once they start taking action, they can't ignore some off-label uses for something that's considered a drug. And while this action started in the middle of 2020, there's no evidence that this decision had anything to do with research around the drug and COVID.
It does seem to work as a hangover preventative provided you take it before starting to drink alcohol. (Somewhere between 15 minutes to a few hours before.)
I think they re-classified it and it's no longer available over the counter. As far as I'm aware it was not banned entirely. I wouldn't know, I stocked up considerably right after the ban, before it disappeared from online stores (but after it already disappeared from Amazon). I figure they wouldn't ban it if it didn't work, like most supplements.
NAC is amazing! We have it available here OTC. I wanted to get in higher quantities from the US but I might not be able to do that, then.
Check this random factoid out: "After about two weeks of 2,400mg NAC supplementation, cigarette usage appears to be reduced voluntarily by around 25%."
I personally take it for my liver. It does wonders to it, similarly to silybilin.
Also note the absence of that sweet/sickly smell on one's breath. This implies a reduced level of circulating acetaldehyde, the primary carcinogen associated with alcohol consumption.
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7649937/