What the list is actually worried about
One thing, mostly. Bleeding under the skin at the moment a needle goes in, and the bruise that results.
Injectable treatments put a needle or a cannula through tissue that is full of small vessels. Nick one and a small amount of blood escapes into the surrounding tissue. That's a bruise, and everything on the pre-treatment list that concerns alcohol, supplements and medication is an attempt to reduce how likely and how large that is.
A second, lesser worry is swelling. Alcohol dilates blood vessels and dehydrates you, and both push in the wrong direction on a day when your face is already going to puff.
Neither of these is a safety emergency for most people. They're outcome and appearance issues, and they matter because you booked this to look a certain way by a certain date.
