Why back leg?, All injections should be on the scruff/neck (whether under the skin or muscle) should they not? all the vets inject in the neck because closer to heart and can access the blood stream quicker, also in a terrier, it is as close as you can get to the infected area i.e. the nose. or am I wrong?
Don't quote me on it, but i'm pretty sure vets do it in the neck as a throwback from jagging "food animals" eg sheep n cattle, so as not to spoil the carcase.