I was shocked to find out NAC historically been made from poultry feathers, which is, I think, why I had a reaction (tons of mucus and runny nose to the extent I felt I was suffocating). After that reaction, I googled what's in NAC and the general answer was poultry feathers. Shocked, I searched further and the best answer I found to what's in NAC is found is on the link below. I would like to find a brand that uses a newer method and doesn't use poultry feathers. But all the brands sold online do not discuss their methods of extraction. Hoping someone here, who's smarter than me, knows how to research NAC products and what method they use.
https://scienceinsights.org/how-is-nac-made-from-l-cysteine-to-pure-powder/