I just squirt the blood out of the syringe. You'll probably bleed on the muscle you hit, so if you switch injection sites you don't want to inject blood into another muscle. If I can't get the blood out, I just reinject the first spot. I don't know if my theory is correct or not but thats what I do. I do know that if you do hit a blood vessel you want to take out the pin and reinject so that you don't inject into your blood stream.