There's little info online about the negatives of NAC, so here's an n=1 to help anyone looking for it
dose: 1200mg (Jarrow NAC sustain), 2-3x daily, for 1-2 months
therapeutic effect: reduce OCD symptoms by ~80% (mind-chatter, intrusive thoughts, minor compulsions like nail biting/tearing), reduce ADHD attention symptoms (moreso as an adjunct treatment, NAC isn't effective by itself but rather reduces the mental jitteriness of stimulants)
side effects: none really big - some vocal hoarseness maybe (would cough up mucus and clears up). occasional nausea but very mild
rebound/dependence?: there is a definite noticeable increase in OCD symptoms as it wears out, and also some increased symptoms on days not taking it (let's say 120% of normal baseline, not unbearable but definitely noticeable). this effect is definitely more apparent vs other nootropics I've tried (ashwagandha, phosphatidylserine)
tolerance: it's only been a bit over a month, but none yet
cheers