The U.S. Food and Drug Administration (FDA) has sent warning letters to five pure powdered caffeine distributors arguing the products are dangerous. The agency said it did so to prevent further deaths from powdered caffeine.

In 2014, two young men who were otherwise healthy died after consuming powdered caffeine. The FDA says it sent the companies the warning letters because “these products are dangerous and present a significant or unreasonable risk of illness or injury to consumers,” the agency wrote in a statement.

According to the FDA, there’s a very small difference between a safe amount of pure powdered caffeine and a toxic amount, and it’s “nearly impossible” to measure safe amounts accurately using normal measuring tools. One teaspoon of pure powdered caffeine equals the same amount of caffeine in 28 cups of coffee, so it’s not possible to use a teaspoon to measure out a standard caffeine serving, the FDA says.

The five companies the FDA warned are SPN, LLC (Smartpowders), Purebulk, Inc., Kreativ Health Inc. (Natural Food Supplements), Hard Eight Nutrition, LLC and Bridge City Bulk. Bridge City Bulk founder Jeffrey Stratton told the New York Times that the company “immediately stopped selling the material” and had not had any complaints.

The federal agency says it is continuing to monitor the powdered caffeine product market and if it finds violations, it will take action, including seizing the product or preventing producers from manufacturing it.

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