I do, I forgot to do it once with some B-12 and it was hard as fuck to get that shit out and it sucked the dose back in when I let go. Yes go ahead and put in what you take out, it's much easier. I know when I use multidose vials I usually poke a 18 to draw with and swap out syringes and I inject whatever dose i'm going to take out in air so when I take off the syringe the stuff doesn't blow out.