Energy drinks — of which there are more than 500 products on the market — represent an $8.3 billion business in the United States, with Monster and Red Bull leading the pack. Energy drink makers “want to promote a fast lifestyle and living on the edge,” an analyst told the LA Times earlier this year. “They want their products to be cool.”

“The term ‘energy drinks’ generally represents a class of beverages that contain high levels of caffeine combined with other stimulants and specialty ingredients,” noted the staffs of three Democrat senators in 2014. “These drinks contain higher concentration levels of caffeine than that of other beverages such as soda.” The report added that about 30 percent of those under the age of 18 report regular use of energy drinks.

A new study published in the Journal of the American Heart Association questions the impact energy drinks have on users’ hearts when compared to other drinks with similar amounts of caffeine. “Little is known about the additional ingredients in energy drinks,” the authors note.

“In line with [energy drinks’] increased popularity is a coinciding rise in energy drink-associated emergency department visits and deaths, which has led to questions about their true safety profile,” they add. Those risks include “adverse cardiovascular events, abnormal heart rhythms such as atrial and ventricular fibrillation, ST elevation, and QT prolongation … However, the exact relationship between energy drinks and these adverse events has gone primarily unexplained.” (QT refers to the heart’s rhythm, and its prolongation can be associated with fatal heart conditions.)

Between 2010 and 2013, more than 5,000 cases of people getting sick from energy drinks were reported to U.S. poison control centers, NBC News reported.

The U.S. Food & Drug Administration sets 400 milligrams a day (“that’s about four or five cups of coffee”) as “an amount not generally associated with dangerous, negative effects,” but notes that it is studying what constitute acceptable levels of caffeine.

To better understand the effects energy drinks have on the heart, researchers found 18 participants via email and flyer recruitment on an a U.S. Air Force base. All the participants were aged 18 to 40 and healthy, and those with several factors that could affect results were excluded. Half of participants were given a 32-ounce commercially-available energy drink, and the other half received a control liquid, with the same amount of caffeine (320 mg) and with other ingredients that weren’t thought to alter heart rates.

The volume of liquid consumed each day correlates to average daily caffeine consumption in the U.S., and the authors noted that about 15 percent of military personnel drink three cans of energy drinks (more than the two cans a day in the study) daily while deployed. After completing a study cycle, participants fasted for 12 hours and didn’t consume any caffeine for 48 hours before switching to the other drink — either control or the energy drink, whichever they hadn’t yet consumed. Of course the containers were the same, so participants didn’t know which they were drinking.

After measuring participants’ heart rates following each drinking session, researchers found a differences in heartbeat cycle and in blood pressure after consuming energy drinks compared to the caffeine control. “Certain populations may consider exercising caution when consuming energy drinks,” they concluded. “Ingredients contained in energy drinks other than caffeine warrant further investigation. Larger clinical trials controlling for the limitations of this study are warranted.”

The study, however, comes with limitations. One doctor told Medpage Today that the study’s small sample size increases likelihood of a “chance” finding. “Study limitations included the absence of a true placebo and that the results only appeared to be significant relative to the caffeine group,” Medpage added.

And the American Beverage Association recently told CNN, “The fact remains that energy drinks have been extensively studied and confirmed safe for consumption by government safety authorities worldwide including a recent review by the European Food Safety Authority. America’s leading energy drink manufacturers voluntarily go beyond all federal requirements when it comes to responsible labeling and marketing practices, including displaying total caffeine content — from all sources — on their packages.”