The thing with b12 is that it is a water-soluble vitamin so if your body needs it, it uses it. If your body has any extra that it can't use, it gets flushed away in your urine. Most people need a lot more than they take in from their diet so you can usually expect some increase in energy. I would think that a b-complex would probably be better in the long run though since if you're deficient in b12, you're probably also deficient in other b-vitamins as well. I have also seen the normal dosage for sedentary folks the same as what Cyphon said, but these dosages seem to be mistakenly applied to athletes which I believe have higher needs. IMO, an athlete would need at least 33-50% more than a sedentary person.