I've never heard of it being called register and i've been working in the ER for 2 years now. You were correct in calling it blood return.
When I was sick, I found a spoon that had leftover residue from a shot a couple days beforehand (yes I know, hardly sanitary, but I was hurting badly) and I know for a fact that 98% of what I actually drew back up through the new cotton was just water. No taste whatsoever. Didn't feel any sort of headache. Not saying you are wrong, I just haven't heard of it, or experienced it myself.
i think register is a slang term, but i'm familiar with it. someone i know who used to do heroin was explaining IV use to me and called it "registering the shot".
i have no idea if it's all that widely used or anything, or what exactly constitutes something as official slang... i was just throwing in that i am also familiar with the term "register" being used in reference to IV drugs. it's also on urban dicitionary
Yeah, i'm sure its called different things in different places lol. So blood return, flash, register...its all the same i suppose.