Ive been taking both Vitamin C and Vitamin B-12 injects for over 5 years.
Both can be taken intramuscularly or subcutaneously as directed/instructed by my GP.
since I already feel like a pin cushion when it comes to shots I go the subcutaneous route and get good results.After the shot I always massage the new injection site to reduce the swelling/bump left from the shot. IF it remains more than a week you might have a problem so monitor it.
As far as the Vitamin C issue goes, Mens Health magazine has an article this month on how C is actually killing cancer cells in lab scenarios(via the injection route-not oral) and animal testing is to begin soon to see if it translates to live beings.
Since the vitamins are water soluble, you always just piss out anything your body doesnt inherently need or use.