Yeah I didn't mention this in the essay, other than briefly alluding to the fact that not all were preventable, but if you look at the footnotes the sources I linked to go into this in more detail. It's difficult to come up with an exact number though: the primary source I'm citing says that (IIRC) only about a third of ADRs are preventable, but the FDA page citing that source says that most ADRs are preventable. And in a way they could both be correct; it's possible that someone could die from taking an emphysema drug that they needed in that moment, but maybe the emphysema itself was preventable.