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.

The Center for Science in the Public Interest, a consumer advocacy group that has petitioned the F.D.A. to ban the sale of pure caffeine, applauded the move but said it hoped that it was a first step toward a ban “and not a substitute for one.”

Laura MacCleery, the center’s regulatory affairs director, said: “People assume something this dangerous would not be sold to consumers in this form. They are used to seeing warning labels and childproof caps on aspirin. And this is just a zip-lock bag.”

Pure caffeine is just one of the energy products that consumer groups have been sounding the alarm about, saying they are riskier than consumers understand and asking the F.D.A. to do more to regulate them.

Ms. MacCleery said pure powdered caffeine was a relatively new product, popping up for sale in some tobacco shops and on the Internet roughly a decade ago. Comparing caffeinated drinks with pure caffeine “is like comparing a table knife and a table saw,” she said.

One of the companies the F.D.A. warned was Bridge City Bulk. The agencycited two products, in packages ranging from one kilogram to 25 kilograms. The 10-kilogram package of caffeine powder contains about 1,230 tablespoons, equivalent to 50,000 servings, it said.

The products, the agency said, “are packaged to contain an amount that would be lethal to many consumers.” It told the company to “take prompt action to correct the violations” and warned that if it did not, the agency could seize its product or stop it from producing more.

In an email, Bridge City Bulk’s founder, Jeffrey Stratton, said the company had “immediately stopped selling the material.” He said it had received “no product complaints at any time, ever.”

The F.D.A. also sent warning letters to Hard Eight Nutrition, Kreativ Health, PureBulk and SmartPowders. The companies have 15 days to respond.

It was not clear if all the companies would stop production, as Bridge City Bulk had, or whether they would include measuring devices or simply change the quantity in the packages. The agency said it would evaluate the responses case by case.

“We do not know how the market will respond to these warning letters and to the concerns that F.D.A. and others have expressed about the safety of pure powdered caffeine, so we cannot speak to whether or not products are coming off the market,” it said.