can be due to a true allergic reaction...this is when antibodies attach to proteins and release histamine. Typically this is dramatic and often involves asthma, wheezing, and swelling of the throat if extreme (some seafood allergies do this, as well as peanuts, and penicillin).

But some drugs cause itching in another way as well, and it is not as life
threatening. Opiates cause histamine release by another mechanism, and this is not a true allergy. This reaction to ALA and NAC may be more like niacin...in that it stimulates histamine release without antibodies.
Many foods do this too... like tomato sauces.

So it is hard to say what is going on here with you, Brian. But one should always be careful. I don't think a doctor would be able to answer you exactly either. Perhaps an allergist would be the best to consult. But a general doctor would not know.