The general advice is to use a broader needle (such as a green 21G) to draw up the B12 and then a slightly narrower one to inject (such as a blue 23G for the thigh muscle).
Personally, I found long needles (orange 25G or grey 27G in 40mm lengths) cause less soreness afterwards than a 23G.
Of course we should use whichever needle we find best but is there any reason not to propose such fine needles in general for most people?
What advantages of a 23G needle am I missing?