A lot of people comment about air bubbles with shooting up. They hear the urban legeond about a nurse having an air bubble int he syringe and it getting caught in your brain. THIS DOES NOT HAPPEN. The reason you do not want any air in the syringe is not because of air getting caught in it, but because when you draw blood to confirm your in a vein, it will cause the blood to start clotting, at some point it will be too thick to push out of the syringe. Plus then you risk injecting a clot into your blood. With 20-40 units that you use for most street drugs, you would beable to inject it faster then it could clot, but if you slip out, or something delays it, your solution could be ruiened. And you waste drug. This is why you want to keep air out of the syringe, not because you will get an air bubble in your brain.