@alleycatkate There is a lot of literature about how NAC can help, but what is missing is literature that describes any adverse effects of an overdose of NAC. What I would recommend for anyone who is taking NAC is to keep track of any changes in symptoms, and stop or consult a doctor immediately if there is an adverse effect. At one time, I read some literature that suggested that an overdose of NAC could have the opposite effect and cause an increase in oxidative stress by causing an alternate biochemical reaction loop. I've been looking, but not finding related literature.

It is from my own experience that I make these suggestions and I have allergic asthma. I did take the same dose that was mentioned in the links from this post NAC 600 mg twice a day, and after a few weeks I did experience my lung function improve and the phlegm was easy to clear, but a few weeks after, I developed thirst that increased every day and became excessive. I wasn't able to tolerate eating protein foods like meat because it increased my thirst. Proteins are made of connected amino acids which digestion breaks down, and NAC also is an amino acid. I stopped taking NAC on the 5th day of the thirst. I was drinking a lot of water around the clock, but couldn't quench the thirst and had to get up several times at night. I called Poison Control and asked about overdoses of NAC, but they didn't have any advice to offer. The next day, there was slight improvement in my thirst, and it actually took 5 days to resolve which was the same amount of time it took to get to that extreme. In all fairness, I was also taking a supplement that was made from cruciferous vegetables which also helps the body detox. It may have been an effect caused by the combination of these. The detox supplement said not to take it if you have gallstones, and what also happened is that the normal amount of bile (which is stored in the gall bladder) was decreased in my intestines. Bile is what colors the end products of digestion, and without it, it is colorless. That symptom is an alarm that something might be blocking a bile duct (which could have been inflammation or gall stones). Everything returned to normal within a few days of stopping the supplement and the NAC. I did see my doctor and had an ultrasound of the bile duct and nothing was wrong. I discussed all of this, but there was no explanation as to why this happened.

Since then I have taken NAC infrequently, but no more than 600mg per day, and not every day. I don't want to do this experiment again. I think there are a lot of unknowns even among health care profressionals about what happens when too much a good thing is too much. There are a lot of benefits of NAC and it is a precursor to glutathione which is a master antioxidant in the body, so it's not a bad thing, but pay attention when you try any protocol for adverse effects. Overdoses of vitamins can be toxic too, and I just wanted to share the information in case someone else has the same experience.

This literature does describe an overdose of NAC that was fatal, but it happened as a medication error in using NAC to treat a patient after acetaminophen poisoning.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4354467/