It's no shocker that those cans of caffeine-packed swill you grab at the gas station aren't the healthiest way to pep up. But more and more evidence reveals how harmful Monster, Red Bull, and other energy drinks really can be. A new research review presented to the American Heart Association in March highlights some pretty scary ways these high-caffeine beverages may hamper heart function.

Looking at data from seven previous studies of healthy people aged 18 to 45, researchers determined that energy drinks can disrupt heart rate and raise blood pressure. First, they looked at QT interval, which measures the time it takes for the heart to rest before taking its next beat. In people who'd consumed one to three energy drinks, QT intervals were noticeably longer than those who didn't indulge. "Prolonged QT interval is associated with potentially lethal irregularity of heart rhythm," says Dr. Gordon Tomaselli, an American Heart Association spokesman. "This can lead to an increase in heart muscle thickness, some forms of heart disease, and even heart failure." The researchers also found that energy drinks raised systolic blood pressure by an average of 3.5 points, which, combined with increased QT interval, could spell more trouble.

But are these real risks for in-shape guys who just want a quick caffeine jolt now and then? "For young, healthy people, this probably isn't a problem," Tomaselli says. "However, it definitely raises a red flag, especially for people who have underlying heart conditions but don't know it."

Tomaselli also says that sipping energy drinks frequently might make you more tolerant to their effects. "You can definitely develop a tolerance to the physiological reaction and need to drink more to get the same lift," he says. "The ultimate danger depends on many factors, but in general, the more you consume, the greater your potential to prolong QT interval, raise heart rate, and possibly have more serious heart problems."

Clearly, with energy drinks, the mantra is moderation. But with all the health benefits of coffee and tea, why not kick the can altogether and get your caffeine from beans and leaves, like nature intended?