Energy drinks can be a godsend when you’re trying to plow through a cram session or an extra crazy day at the office. But some might contain so much caffeine that you’re putting your health in serious danger. At least, that’s the message from World Health Organization officials, who warn that increased use of energy drinks is dangerous due to too-high levels of caffeine and other stimulants.

Wait, what? Yes, caffeine in moderate doses helps you feel revved and alert, and getting a little too much can make you feel jittery. But going overboard can actually put you at risk for caffeine toxicity, resulting in heart palpitations, high blood pressure, nausea and vomiting, and even seizures.It’s a lot easier to go overboard with energy drinks than with coffee.

While some energy drinks offer roughly the same amount of caffeine as an eight-ounce cup of java (about 95 mg caffeine), there are plenty that deliver twice that amount, found a 2012 test by Consumer Reports. (And that’s per serving, not per bottle.) There’s also the fact that the caffeine in energy drinks can flood your system faster than the caffeine in coffee, since we tend to guzzle the former while slowly sipping the latter. And since we’re more likely to down energy drinks before exercising, they can pose the added threat of dehydration.

“A lot of energy drinks are marketed as sports drinks, but caffeine is dehydrating, and they don’t have the hydrating ingredients that something like Gatorade does,” said registered dietician Ellen Albertson, Ph.D., R.D.N.

That’s not all. To give you an extra jolt, energy drinks often combine caffeine with other stimulants like guarana, ginseng, gingko biloba, l-carnitine, and yerba mate. And even though the FDA considers all of those individual ingredients to be safe, experts don’t know for sure what the effects are when the uppers are consumed together in high doses, explained US Pharmacopeia senior scientific liaison Carla Meija, Ph.D. But clearly, they aren’t good: emergency room visits involving non-alcoholic energy drinks doubled between 2007 and 2011, and most involved adverse reactions. (The majority also occurred among adults ages 18 to 39, not teenagers.) Several people have even died after consuming numerous energy drinks.

Still, none of this means that you need to swear off your Monster or your Red Bull altogether. While everyone processes caffeine differently, consuming around 400 mg per day seems to be okay, explained João Breda Ph.D., from the WHO Regional Office for Europe. Be mindful of how much you’re taking in by reading nutrition labels (to be safe, skip any drinks that don’t list caffeine content), and remember to account for other caffeinated things you’re taking in, like coffee. And even if you’re zonked, try not to jolt your system by taking in an entire day’s worth of caffeine at once. “Stick to 100 mg in one shot,” Albertson advised.

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