When I started B 12 injections my pharmacist asked if I wanted the big needles or the little ones. I thought he was joking, and he was a little, but he explained there were two sizes. I, of course, chose the smaller [shorter]. When I went back for more from a different pharmacist I was given the bigger ones. It's intimidating enough injecting onself with the smaller needle, but the second pharmacist explained that the longer one improves absorption.
I went back to the first pharmacist and he said if I had big muscles [I don't] the longer one might make a difference but in my case it wouldn't.
However, twice the doctor's office has called following a blood test [just shy of 400]and told me to do a "reload" which is to inject every week for a month and then resume monthly shots.
What size do you use?