Drinking caffeinated energy drinks can kill you, according to a new report from the FDA. The findings linked 5-Hour Energy, a two-ounce shot marketed to adults that contains more than 100 milligrams of caffeine per ounce, to 13 deaths, as well as to major health problems like convulsions, miscarriages, hemorrhage, and deafness. (For comparison, coffee and Red Bull each contain about 20 milligrams of caffeine per ounce.) The FDA also cites Monster Energy, a 16-ounce energy drink, for five deaths and incidences of heart attack, severe diarrhea, and arrhythmia. The problem with these drinks, says Dr. John Higgins, a leading researcher on the topic, is that they contain excessive synthetic caffeine – created in a lab, not found naturally in plants – along with compounds like taurine and guarana, about which there’s been very limited research done. “Most energy beverages are unregulated by the FDA, so they have not gone through the rigorous testing required for most food and drugs that we use each day,” says Higgins. He says that people should stick to coffee or tea instead.