I have inch and a half needles. The one time I accidentally used a shorter needle, I lost quite a bit of T so never did that again. Also important is the thickness of the needle. The doctor kind of looked at me strange for preferring super thin needles to the freaking huge needles that come with the syringe, because apparently thin makes it hard to push the T out...never been a problem for me though, and it hurts 20x less and barely bleeds.