It's not the NAC, but the mercury I believe. It is highly risky to give anybody large doses of something like this when they are very mercury toxic. Unless they respond well to it. Either way, starting with small doses would be vital, unless a person wishes to risk a reaction that they may regret.

Products like NAC, cysteine, methionine, garlic, chlorella, MSM, cilantro etc can be problematic for high mercury toxic people. Others maybe lucky and do ok. The risks for those that don't are high. Suicidal thoughts can be a result of wild mobilisation of mercury into more sensitive tissue - brain included.

Even consuming high sulfur containing foods can be enough to encourage a reaction from mobilising mercury. When I had a high body count of mercury, I remember onions, eggs and cabbage etc used to bring it on. I used to feel repoisoned and quite ill - but certainly nothing to the extent of how I felt using the supplements listed above. At least with the food, though it's not always pleasant, it was somewhat more manageable than the other. Though one should not expect to feel ok when mercury is being moved/removed regardless of what they use. Since it's a toxin, it will indeed provoke symptoms as it's trying to come out. Usually a person will feel worse and often quite horrible/sickly. After this, there is often a feeling of improvement. However, due to the nature of mercury as it's trying to come out, you get a clearer patch and before you know it, you feel worse again as more surfaces.

I do not really understand why some people do ok on those products and others have horrible (even quite dangerous) side effects. Just emphasizes the fact we are all unique.