> It just has to be delayed. Like many years after application.

That's one thing. In this case, I don't really know if it's possible to test for something like delayed effects. I'm not even sure if you can identify them with 100% certainty; if you can prove that these effects come from this particular drug and not from another one.

> Or trigger on very specific and rare circumstances. Not likely in a trial, but near certain at a population scale.

And this is different thing. "Specific and rare circumstances" will not lead to millions of deaths (I apologize if I’m being too nitpicky about this particular phrasing, but I want to speak specifically in the context of “millions of deaths”). “Specific and rare circumstances” occur even with fully effective and "proper" medications - this is called “contraindications.” But such rare cases, as I’ve already said, will not lead to mass deaths - precisely because they are rare. I apologize again for focusing on the "millions", but please don’t confuse the scale of the problem.