The man, who was in his 50s and otherwise healthy, showed up at a hospital after the entire left side of his body abruptly went numb and he was left with clumsy, uncoordinated muscle movements (ataxia). His blood pressure was astonishingly high, at 254/150 mm Hg. For context, a normal reading is under 120/80, while anything over 180/120 is considered a hypertensive crisis, which is a medical emergency.

  Four weeks into his recovery, he was on five different drugs to try to bring down his blood pressure. At that point, doctors pushed for more lifestyle information from the man, who finally revealed that he had a habit of drinking an average of eight high-potency energy drinks every day.

  Each of the 16-ounce drinks was labeled as containing 160 mg of caffeine, a stimulant that can raise blood pressure. For reference, there’s about 90 mg of caffeine in a normal cup of coffee. So eight of these energy drinks would be 1,280 mg, the equivalent of a little more than 14 cups of coffee a day. But the doctors point out that the labeled amount of caffeine was only the “pure caffeine” amount. Such energy drinks contain additional ingredients, like guarana (a plant native to the Amazon used as a stimulant), that can contain “hidden caffeine.” Guarana is “thought to contain caffeine at twice the concentration of a coffee bean,” Coyle and Munshi write.