Federal regulators issued guidance Friday making it clear that it is illegal to sell pure and highly concentrated caffeine in bulk to consumers.

The Food and Drug Administration (FDA) said it’s prepared to remove these dietary supplements from the market, after reports of at least two deaths in otherwise healthy individuals.The products “present a significant public health threat” because of the high risk consumers will mistakenly use them at dangerous doses, FDA said.

Previous FDA actions including warning letters have failed to stop the flow of these highly concentrated or pure caffeine supplements being sold directly to consumers online. They’re sold in bulk with up to thousands of recommended servings per container.

“We know these products are sometimes being used in potentially dangerous ways,” said FDA Commissioner Scott Gottlieb, a physician. “For example, teenagers, for a perceived energy kick, sometimes mix dangerously high amounts of super-concentrated caffeine into workout cocktails.”

A half cup of a highly concentrated liquid caffeine can contain approximately 2,000 mg of caffeine, while a single teaspoon of a powdered pure caffeine product can have about 3,200 mg of caffeine. That’s up to 28 cups of coffee, which is a potentially toxic dose of caffeine.

In fact, less than two tablespoons of some formulations of powdered, pure caffeine can be deadly to most adults, while even smaller amounts can be life threatening in children. Risk of overuse and misuse is high when highly concentrated caffeine is sold in bulk and consumers are expected to measure a very small, precise recommended serving.

The recommended safe serving of highly concentrated or pure caffeine products is often 200 mg of caffeine, which equates to 1/16 of a teaspoon of pure powder or approximately 2.5 teaspoons of a liquid. Consumers often don’t have the right tools to correctly measure such a small amount. Even if they do, simple and common errors, such as packing the powder too tightly or using a “heaping scoop” instead of a “level scoop,” can increase the amount of caffeine in a single dose, FDA says.

Dietary supplements containing caffeine in other forms aren’t likely to present the same safety risks, especially if they’re sold in premeasured packets or containers, in solid dosage forms such as tablets or capsules, or when they aren’t highly concentrated.

In 2015 and 2016, the FDA issued warning letters to seven distributors of pure powdered caffeine, with several of the letters citing that the products were dangerous and presented a significant or unreasonable risk of illness or injury to consumers.