I've looked into this before. All the research said was it has the same possible side affects as coffee, well, I've been drinking coffee with no adverse reaction for 20 yrs. I think I'll be ok!
And that it has large amounts of B vitamins and Niacin. Like 1000% percent of your daily allowance! That Niacin flush is what people freak out about, but Niacin is good for you. I found nothing that said either one could be harmful if taken in those dosages. And they are water solluable, so what your body doesn't use is flushed away if you drink alot of water. It's only a problem if you take more than 1 a day.
I've gone so far as to look up every ingrediant in Monster Energy drinks. The Tuarine, Guarine, etc. That stuff is not bad for you! They have good properties, such as lowering cholesterol, etc.
There is a ton of sodium and a ton of sugar. It's not a health drink. But it's a great tool when I need to do my saturday cleaning. As with anything, you don't want to go overboard. You can do just as much damage if you drink a whole lot of diet coke! You can't go nuts on it, but it's fine as needed, no more than 1 a day.