I aspirate as it only takes a second... But, usually if I am deep in a quad muscle, I cannot even pull the syringe up. I had blood fill the syringe once--and The syringe pulled up easily as a result. So, yes--I aspirate. It's an extra step that makes the injection take a bit longer. But, I would rather shoot into the muscle as opposed to a vein. I'm not sure the needle tip was actually in the vein, but it could have been--thus the reason for aspirating.