M.K.
I would suggest taking a B-12 suppliment to determine its effects before getting a shot... ask your doctor if he even thinks you need the shot. Do you have a B-12 deficiency? Do you have other deficiencies or health problems that could be causing you to have low energy?
There is almost never a magic pill to fix your one problem, but there is often a cause for the problem, and you can work to improve that.