I don't see caffeine. As either effective or efficient, particularly in the high doses found in energy drinks. It is a stimulant. In mild doses, equivalent to a couple cups of coffee, it is generally no big thing, but even then some people develop anxiety, tremor and insomnia with any amount of caffeine. Energy should come from good health, good diet, exercise and adequate sleep, not from stimulants including caffeine.
Don't drink too much. Of it.