You definitely need to be careful with energy drinks. The more I read about them, the more the negatives stack up. I've posted a bunch of articles on my blog (click on my name to the left, and then go to the News & Info section) that reference other websites that have discussed the ills of energy drinks. In fact, Health Canada recently recommended children/teens not drink them at all, period. Interesting.
Energy drinks bear higher percentage of caffeine than other caffeine sources like tea and coffee. They even contain more caffeine than other soft drinks as well.