I draw up with an 18 g and inject with a 21 g, 1". I'm (almost) an RN and there's no way I would use the same needle for drawing up and injecting my patients. Like the others have said, infection and pain are the two biggest reasons for switching needles. Plus, T is fairly thick and drawing up with a tiny needle is difficult. Unless you like your shot to be time consuming, I'd use a different needle.