Energy drinks promise a boost, a buzz, even to give you wings. Most contain a mix of caffeine and the stimulant guarana, along with a variety of other vitamins and nutrients. Manufacturers in this small but growing slice of the beverage market say the drinks are no more dangerous than a strong cup of coffee. But some watchdog groups and federal agencies have raised questions about how they're marketed, transparency regarding ingredients, and safe use. We consider the rising popularity and questions surrounding energy drinks.