I've been getting many IM injections in my arms. I usually alternate arms but overall my left arm has received more injections. I was not told by anyone that I needed to be careful about rotating where I get the injections done. Now, I have decided to get them done subcutaneously because my arms get more and more painful after any injection/vaccination in the arm and take a long time to heal. I am afraid I may have scar tissue there or delayed healing.
So if you get IM injections - the standard at my clinic, be careful not to get in the same place too many times. I think the arm isn't a good place to get the b12 shots.