That you cannot imagine an approach doesn't mean an approach doesn't exist. It may be possible to form a generalized understanding of an individuals predisposition based on testing with other substances, just as one example.
You’re still taking a substantial risk when forming that generalized understanding, and then another substantial risk when you start making conclusions from that general understanding.
Ok, so basically, since we haven’t proven the opposite, we can’t be sure, regardless of how the situation appears. I think that’s a weak argument in a general sense, but point taken.
No, I'm not arguing that proof is necessary for certainty. I'm pointing out that the implied reasoning is akin to argument from ignorance. We don't have to take a position on a topic if we are ignorant, but we often do so when doing so can be used to justify existing biases.
And for what?