Energy drinks are the fastest-growing part of the beverage industry. In the US, sales are reaching more than $10 billion in 2012 - more than Americans spent on iced tea or sports beverages like Gatorade.
Their rising popularity represents a generational shift in what people drink, and reflects a successful campaign to convince consumers, particularly teenagers, that the drinks provide a mental and physical edge.
But the drinks are now under scrutiny by the US Food and Drug Administration after reports of deaths and serious injuries that may be linked to their high caffeine levels.
However, one thing is clear: the energy drink industry is based on a brew of ingredients that, apart from caffeine, have little, if any benefit for consumers.
"If you had a cup of coffee, you are going to affect metabolism in the same way," said Robert W Pettitt, an associate professor at Minnesota State University, who has studied the drinks.
Energy drink companies have promoted their products not as caffeine-fuelled concoctions but as specially engineered blends that provide something more.
For example, producers claim that "Red Bull gives you wings," that Rockstar Energy is "scientifically formulated" and Monster Energy is a "killer energy brew."
Promoting a message beyond caffeine has enabled the beverage makers to charge premium prices.
A dearth of evidence underlies such claims. Only a few human studies of energy drinks have been performed and they point to a similar conclusion, researchers say - that the beverages are mainly about caffeine.
"These are caffeine delivery systems," said Roland Griffiths, a researcher at Johns Hopkins University who has studied energy drinks.
A scientist at the University of Wisconsin became puzzled as he researched an ingredient used in energy drinks like Red Bull and Monster Energy.
The researcher, Craig A Goodman, could not find any trials in humans of the additive glucuronolactone that is related to glucose.
He eventually found two 40-year-old studies from Japan that had examined it and found it made lab rats swim better.
"I have no idea what it does in energy drinks," he said.