The U.S. Food and Drug Administration on Tuesday issued warning letters to five distributors of pure powdered caffeine, as the federal agency stepped up efforts to curtail ingestion of the stimulant in its pure form.

The drug regulator had in 2014 issued a consumer advisory warning of the risk to taking pure powdered form of caffeine after two men died from overdose.

Earlier in 2012, the FDA issued a warning letter to Breathable Foods Inc, a company that marketed an inhalable caffeine product as 'breathable energy'.

The agency had in the same year investigated reports of five deaths that may be associated with Monster Beverage Corp's namesake energy drink that contains caffeine. (reut.rs/1fTme2j)

Caffeine is commonly used in small and safe doses in products such as coffee, carbonated beverages and energy drinks.

Common milder side-effects from taking increasing amounts of caffeine include nervousness and tremors. Pure powdered caffeine taken in large doses, however, can cause erratic heartbeat, seizures and death.

One teaspoon of pure powdered coffee was equivalent to the amount of caffeine in about 28 cups of regular coffee, the FDA said on Tuesday. (1.usa.gov/1hR6nTP)

The five distributors issued warning letters were marketing pure powdered caffeine as a dietary supplement, the agency said.