Just a little bit. You'll know when you pull it back just that little bit if you're in a vein or not. If you're in one, it'll be a little easier to pull back and you'll see blood come into the syringe. If you're clear, then you'll see an air bubble come into the syringe. That's good, you want the tiny air bubble... You're good to push if you see one. If not, take it out and put it in a new location.