B12 injections are great if you can not assimilate the oral version. I would discuss this with your physician. Using it as a form of "energy" is not a great idea though. B12 is good for energy, yes, but, it is also good for many other things and I would not recommend it just to loose weight.
If you want more energy, go to your local health food store and look into some of their "power" drinks, not the ones that have "speed" in them or alcohol, but the ones that have the nutrients that build up your system with vitamins, minerals, herbs and amino acids.
The question is meaningless, because there are various factors, including the medical history and weight of the patient. Ask your doctor.
That being said, sublingual B-12 is quite effective (though not as effective as the injections), and there is no good reason to have the injections.