How much would you pay for a drink that promises to revitalize your mind and body, elevate your energy or give you the focus you need to cross every item off your to-do list?

Catching a third wind is priceless, which is why energy-drink sales are in the billions each year. But there may be a hidden cost.

As sales skyrocket, so do energy-drink-related emergency room visits: Between 2007 and 2011, they doubled from about 10,000 to nearly 21,000 visits. In a recent study from the University of Southern California – which randomly sampled emergency room patients – a third of patients who reported downing energy drinks had adverse reactions ranging from heart palpitations to chest pain to seizures.

Even more frightening, the U.S. Food and Drug Administration has received reports of at least 15 deaths related to energy drinks. Although there’s no proof linking these negative reactions to drinking an energy drink, “no one really knows how dangerous they are,” said Michael Jacobson, executive director of the Center for Science in the Public Interest in Washington, D.C. “They certainly raise a caution flag about drinking too much (caffeine) too quickly.”

The FDA recently launched an investigation into the safety of caffeine added to food and beverages.

Contrary to what you might think, these purported power drinks aren’t just being guzzled by frat boys. The largest increase in energy-drink-related ER visits was in people over 40.

Why? Energy drinks may deliver an instant jolt that java doesn’t.

“Some contain large amounts of caffeine. And they’re consumed differently than coffee. They’re are gulped, rather than sipped,” said Jacobson.

And that’s where the problem lies.

“In the ER, we’ll usually see somebody who has heart palpitations or maybe hand or body tremors from drinking too many. However, we have seen a few patients who’ve had seizures,” said Dr. Sean Nordt, director of toxicology at USC’s Keck School of Medicine, Department of Emergency Medicine.

These reactions aren’t imaginary, either. A new, unpublished, study presented at the American Heart Association’s 2013 Scientific Sessions found that downing energy drinks raises systolic blood pressure by an average 3.5 points and can cause potentially fatal irregular heartbeats. Your risk of dangerous heart-rhythm disturbances is higher if you have an underlying heart condition or high blood pressure.

The most active ingredient in energy drinks is caffeine. While some brands have just 50 milligrams, others pack as much as 215 mg. To put that in perspective, an average 8-ounce cup of coffee contains roughly 100 mg and most experts advise limiting your caffeine to 400 mg a day, max.

Even more troubling, you may not be able to find out exactly how much caffeine is in your can. Energy-drink manufacturers are not required to list the amount of caffeine – whether it’s marketed as a beverage or as a dietary supplement – unless it’s added in the form of pure caffeine.

Almost every energy drink has its own “energy blend” of proprietary ingredients. If a drink’s caffeine comes in the form of coffee, tea or another natural caffeine-containing substance, such as guarana, you might not know the total amount. So even if a can said it contains 200 mg of caffeine per serving, it may deliver significantly more.

Energy drinks can be dangerous on their own, yet they’re even more perilous when combined with alcohol. And that pairing has gone mainstream: bars serve up a new breed of cocktails like “1.21 Gigawatts,” a concoction of Red Bull, raspberry vodka, cognac and grenadine. When you mix alcohol and caffeine, their effects blunt one another.

“The caffeine makes you think you’re sharper than you are,” said Dr. Kent Sepkowitz, vice chairman of clinical affairs in the department of medicine at Memorial Sloan-Kettering Cancer Center in New York City. “But your judgment is still that of a drunk person.”

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