WASHINGTON — The Food and Drug Administration said Tuesday that it had sent warning letters to five producers of pure powdered caffeine, in what consumer advocates said was a long overdue move against the product and the industry that makes it.

In its pure form, caffeine is powerful. A teaspoon of caffeine powder is roughly equal to 28 cups of coffee, and a tablespoon can be lethal. A 100-gram package, about 3.5 ounces, can have as much caffeine as 400 “tall” cups of Starbucks coffee, 1,250 cans of Red Bull or 3,000 cans of Coke. Last year, two otherwise healthy young men died after using too much, prompting the agency to warn of the potential dangers.

The agency said Tuesday that the product, which is often mixed into beverages, was “potentially dangerous” and presented a “significant or unreasonable risk of illness or injury.” Because of its potency, safe amounts of pure caffeine are very small and therefore hard to measure. Kitchen measuring tools like teaspoons are not precise enough, the agency said, because of variations in how tightly the powder is packed.

“The difference between a safe amount and a toxic dose of caffeine in these pure powdered products is very small,” the agency said on its website. Identifying a safe amount of pure caffeine requires a precise scale, it said. Too much caffeine can lead to a rapid or dangerously erratic heartbeat, seizures and even death.