Powdered caffeine supplements sold in the United States can be deadly, says a new warning from the U.S. Food and Drug Administration (FDA). The agency recently issued warnings to five different makers of the stimulant about the safety of their products and they way they are marketed, NBC News reports. Powdered caffeine is a hyper-concentrated form of the stimulant that companies sell to people who want to make their own energy drinks. These sales are legal, and the stimulant is commonly purchased online and in stores across the country. But ingesting it isn’t as harmless as sipping a cup of strong joe, cautions the FDA. Just one teaspoon of the powdered stimulant can deliver as much caffeine as 28 cups of coffee. The FDA reported that, to date, powdered caffeine supplements already killed two teens. The agency issued a warning about powdered caffeine supplements in 2014 when an Ohio teen died after using the stimulant. Health experts warn that caffeine overdoses can cause a myriad of health issues, including nervousness, tremors, vomiting, diarrhea dangerously erratic heartbeats, seizures, and even death.