Some pure caffeine products are now unlawful, according to the Food and Drug Administration.

The agency said the ban focuses on liquids and powders containing pure or highly concentrated caffeine, which are often sold in large tubs containing thousands of servings — not everyday products like coffee and energy drinks.

One teaspoon of pure powdered caffeine is equivalent to 28 cups of regular coffee, the FDA says. A recommended safe serving size of pure caffeine products is usually 200 milligrams, or 1/16th of a teaspoon of powder and about 2½ teaspoons of liquid. Amounts that tiny can be tough to measure, and slight variations can be deadly.

Two healthy young men, an 18-year-old and a 24-year-old, died from caffeine overdoses in 2014. Their families met with the FDA, which then started warning consumers about concentrated caffeine products.

In 2015, the FDA issued warning letters to five distributors of pure powdered caffeine: SPN's Smartpowders, Purebulk, Kreativ Health's Natural Food Supplements, Hard Eight Nutrition and Bridge City Bulk. In 2016, it issued two more to Global Marketing Enterprises and ALV Supplement Direct.

"Despite multiple actions against these products in the past, we've seen a continued trend of products containing highly concentrated or pure caffeine being marketed directly to consumers as dietary supplements and sold in bulk quantities, with up to thousands of recommended servings per container," FDA Commissioner Scott Gottlieb said in a statement. "We know these products are sometimes being used in potentially dangerous ways."

Advocates and lawmakers have called on the FDA to ban concentrated caffeine products altogether. The guidance issued Friday doesn't go quite that far, instead focusing on pure and highly concentrated caffeine sold in large amounts and can't be easily measured.

For example, pills or tables and premeasured packets would be acceptable. So would bulk amounts, as long as they were diluted enough that a measurement error wouldn't make them toxic.

The changes go into effect immediately, meaning the FDA can take steps right away to start taking illegal products off the market.

"We're making clear for industry that these highly concentrated forms of caffeine that are being sold in bulk packages are generally illegal under current law," Gottlieb said. "We'll act to remove these dangerous bulk products from the market."