I think it comes in two packages, known and unknown. We've historically thought of adverse events as something down the road that was unexpected in chronic use and wasn't predicted from the initial studies. That's one category.
The major category, as far as the physician is concerned, is the untoward effect you can expect from the initial studies, which can occur if you go beyond a certain dose.
But as Jim says, the other adverse effect is failure to respond. So those are the three categories: known, unknown and unexpected, and failure to respond.