Just got the run down from a family member that got it for me. If you are low on b-12 you will find that your appetite will be low along with low energy. The injection form is many many times stronger of course and the liver will take it in and hold it till it is needed.
From what I understand once you are up to normal level you can maintain that level by taking oral b-12. I started it and within 2 injections I felt better and was 100% hunger then the days before.