The U.S. Food and Drug Administration said Friday it is banning the sale in bulk volumes of dietary supplements that contain high levels of concentrated or pure caffeine after at least two deaths of otherwise healthy individuals.

The regulator said it is issuing new guidelines that are effective immediately and that make such supplements, which can be purchased in liquid or powdered form in large quantities in stores and online, unlawful.

“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,” the FDA said in a statement. “We know these products are sometimes being used in potentially dangerous ways.”

Teenagers looking for an energy boost, for example, are inclined to mix high amounts of powdered or liquid caffeine into drinks they take before working out. But while a half cup of concentrated liquid caffeine can contain about 2,000 mg of caffeine, a single teaspoon of powdered pure caffeine can contain 3,200 mg. That’s equal to about 20 to 28 cups of coffee, a potentially toxic dose.

“Risk of overuse and misuse is high when highly concentrated caffeine is sold in bulk quantities, and consumers are expected to measure a very small, precise recommended serving,” said the statement. “Regardless of whether the product contains a warning label, such products present a significant and unreasonable risk of illness or injury to the consumer.”

Consumers are often directed to take micro-doses of such supplements, up to 1/64 of a teaspoon, which is impossible to measure without special equipment.

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It’s not the first time the regulator has cracked down on companies that make caffeine products. In 2015 and 2016, it send warning letters to seven distributors of pure, powdered caffeine, saying the products were dangerous.

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In February, the FDA said the supplement kratom, an Asian plant-based dietary supplement that’s been touted as an aid for opioid withdrawal, pain, anxiety and depression, is an opioid.

Americans drank 88.8 gallons of coffee in 2016, according to Euromonitor International, an independent provider of strategic market research, as MarketWatch’s Kari Paul has reported.

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