Another example: it has been known that many substances do prevent, but if you already have cancer, than your condition will worsen.
Back to NAC: it is healthy for you in the typical doses which is about 200-1000 mg I would say. If you take acetaminophen on the daily, then it is especially recommended.
In any case, I do not recommend taking it long-term, or not without breaks in-between. Take it for 2 months, stop for a month, take it for another 2 months, then stop for a month and so forth. You can do 1 month on then off, and so on. Up to you.