The FDA has received reports of 13 deaths and 33 hospitalizations over the past four years, claiming 5-Hour Energy was possibly involved.
"If someone was to drink multiple cans, which is when we really see a lot of the side effects with this, you're getting up to astronomical amounts -- 30, 40 cups of coffee ," said Dr. Sean Patrick Nordt, USC director of the section of toxicology.
It is the second time in a month the FDA is investigating claims that energy drinks are causing fatal reactions. In October, Monster Energy, another highly caffeinated energy drink, was linked to five deaths.
But according to the manufacturers, the claims have no proven link between the drinks and the deaths. In a statement, 5-Hour Energy said the product is "intended for busy adults." The company also said its compact product contains "about as much caffeine as a cup of the leading premium coffee."
But a recent Consumer Reports story found that 5-Hour Energy had caffeine levels as high as 215 milligrams. Compare that to a cup of coffee, which can range from 75 to 150 milligrams.
What could be key to this investigation is the type of caffeine used in the energy products. While companies compare their caffeine levels to coffee or tea, the caffeine in a regular cup of coffee is naturally occurring and has natural inhibitors. But energy products contain anhydrous caffeine, which means it's dehydrated and may act differently in the body.
"It has no inhibition, so it is essentially a very strong effect upon a central nervous system," said public health expert Chris Noonan.
He says anhydrous caffeine is found in all kinds of products like sodas, drinks, even over the counter medications and more in various doses.
In addition to this latest probe by the FDA, energy drinks are also coming under the scrutiny of lawmakers who want to limit the amounts of caffeine in these shots and beverages. That's partially based on a report that emergency room visits involving energy drinks jumped 10 fold from 2005 to 2009.