FDA last month warned two companies they were illegally selling highly concentrated caffeine products considered a threat to public health.

FDA sent the warning letters to Liquidcaffeine.com and Dual Health Body and Mind after publishing related guidance in April for the dietary supplement industry.

In the guidance, FDA said products containing pure or highly concentrated caffeine in powder or liquid forms are considered unlawful when sold in bulk amounts to consumers, due to the significant health risks they pose.

FDA has cautioned it is virtually impossible to accurately measure a safe amount of powdered bulk caffeine with common kitchen measuring tools, such as a teaspoon. A single teaspoon of powdered pure caffeine is potentially toxic, containing the equivalent of up to 28 cups of coffee, according to FDA.

The warning letters were dated May 29, 2018, but FDA didn’t announce its actions until June 5.

“Despite being informed of the dangers of highly concentrated and pure caffeine, we’re still finding companies that are disregarding consumer safety by illegally selling products with potentially dangerous and lethal amounts of caffeine,” FDA Commissioner Scott Gottlieb, M.D., said in a statement announcing the letters.

Liquidcaffeine.com and Dual Health Body and Mind, whose products targeted by FDA are sold as dietary supplements, did not immediately respond to requests for comment.

Several trade associations have supported FDA’s position, including the United Natural Products Alliance (UNPA), which in 2015 adopted a no-sale” policy for bulk powdered caffeine for retail sales. UNPA recently adopted a similar no-sale policy for liquid, high-dose caffeine products for retail sales.

In a memo to his members, UNPA President Loren Israelsen said FDA’s recent warning letters confirm “the agency is serious about this issue, and we remind our members of the importance of abiding by our no-sale policy which, to our knowledge, all members currently respect.”

FDA’s guidance is neither legally binding nor does it signal that caffeine in dietary supplements is prohibited under all circumstances.

“When formulated and marketed appropriately, caffeine can be an ingredient in a dietary supplement that does not present a significant or unreasonable risk of illness or injury,” FDA explained in its guidance.

For example, FDA stated it doesn’t expect the following products to be adulterated supplements, assuming they comply with all applicable legal requirements: