and there’s zero record of whether they have any drug allergies or intolerances
Nowadays, you can test for anesthetic drug resistances with DNA site testing, (as in, a blood test), though of course, this does not mean that it is always used, for various reasons.
A personal anecdote:
I’ve had one done after I had bad side-effects to ADHD medication I had tried, and I haven’t had any bad effects in the medications I’ve tried since. (besides drowsiness)
Edit: I want to say, I’m not saying that getting the dosage right is not incredibly difficult, I just wanted to mention that we can detect drug allergies without putting them under, (so to speak).