Well, for one thing it is a nutritional supplement that is not subject to FDA standards. Which means that when you buy it, you have absolutely no assurances that what's in the bottle is NAC in the first place.
Proven? To the best of my knowledge there is not a single reliable and verifiable randomized controlled blinded study published in a juried scientific journal supporting your claim.
What will stop
@Thongjy's paranoia and give him a sigh of relief is some solid fact-based information from a reliable source.