According to this NSFW clip of David Cross as Todd Margaret, yep, pretty dangerous:
[youtube http://www.youtube.com/watch?v=kfPReIAKxeQ]A reader responds to the previous post with his professional opinion:
I have about one referral per year to my cardiology practice where the patient had acute and severe symptoms after consumption of a high-caffeine “energy” drink. I’ve also seen them sitting in my clinic room about to shake themselves silly with tremor and palpitations. These patients have had to be hospitalized for symptoms on at least one occasion, and you might be interested to know that in some of my cases these people work as long-haul truck drivers on the interstate highways. I get that you can ingest a high daily quantity of caffeine by going to Starbucks. Please consider, however, that the people I see are downing their energy drinks after they have already ingested their normal daily quota of coffee or tea – it is not one or the other.
The Dish 