Energy drinks contain up to five times more caffeine than coffee, but the boost they provide is fleeting and comes with unpleasant side effects like nervousness, irritability, and rapid heartbeat, says Goodson. Plus, energy drinks often contain high levels of taurine, a central nervous system stimulant, and upwards of 50 grams of sugar per can (that's 13 teaspoons worth!). The sweet stuff spikes blood sugar temporarily, only to crash soon after, leaving you sluggish and foggyheaded—and reaching for another energy drink.