I am going to do my best to explain this given my limited medical background, maybe P 22 could correct me if I mistep.
B12 is a water soluable vitamin - one that is very efficiently stored by the body. Most adults have an extremely long supply built up (years upon years) so if their body stopped absorbing the vitamin they would not notice for a long time. So it goes to say, that it would take a long time for noticeable syptoms of deficiency to appear.
Deficiency begins when the body stops producing a stomach molecule called Intrinsic factor. Intrinsic factor aids in the absorption of B12 into the blood stream. I believe there may be other ways of being B12 deficient such as being malnourished (but chances are that isnt a problem for most on this board).
Given the solubility and the bodies ability to store the vitamin, if you are not deficient the body most likely is not able to absorb any surplus introduced artificially. However, if you are deficient that is probably why the increased hunger and energy is coming from.
I had a long debate with one of buddies who is in his residency about the viability of using injectable B12 and if there are any tests that could be done. He told me that there are tests (I believe for stomch pH levels - although I may be wrong) but the cost of the test makes it not worthwhile at this point in life, especially if the sole reason I was doing it was for bodybuilding purposes. He said I would be better off just trying it to see if it makes me feel better.
Anyway this is how my friend explained to me. Take it for what its worth, but I was also interested in it at one time, and have decided not to use it. I just saw it as throwing my money out the window at my age. Anything beyond my lay attempt to explain everything would have to be answered by P22, spongebob, or your doc.