Was told by my doc to avoid caffiene but a cup would be okay if taken at least 30 to 45 minutes after phen. I have to say, however, I, myself, would never consume any energy drinks while taking phen. In my honest opinion, NRG drinks are like heart attacks in a can. They are loaded with all kinds of ****, caffiene, and SUGARS. Not very good for someone trying to diet! I don't allow my teen to drink them anymore, so I don't have ingredients and calorie counts in front of me.