A new study also suggests that the drinks may be putting soldiers at risk. Forty-five percent of American service members in combat units in Afghanistan consume energy drinks every day — many drinking them for free in military cafeterias. The 14 percent who drank at least three a day were more likely to sleep under 4 hours per night, report stress-related sleep problems, and fall asleep on guard duty than those who drank less, the study showed.

Experts say it’s also possible that other ingredients in caffeinated energy drinks or the combination of those ingredients with caffeine are causing or contributing to the reported health problems.

“You have to look at a product in its totality,” said Gerry David, president and CEO of Celsius, another energy drink company. “Things work differently when they’re together.”

Celsius contains no sugar — though many other energy drinks are loaded with it — and has been shown safe in seven separate studies, David said.

It’s possible that the risk increases when energy drinks are mixed with other substances. Combining alcohol with energy drinks has been shown to be dangerous. In 2010, the Food and Drug Administration sent warning letters to four companies that the caffeine added to their alcoholic beverages was unsafe. The companies removed the caffeine and now market their drinks as alcoholic, rather than energy products.

The federal Drug Abuse Warning Network reported last year that 7,301 emergency department visits were connected to caffeinated energy drinks between 2004 and 2009. Of those, 44 percent had alcohol, medications or illegal drugs in their bloodstream in addition to the energy drink..

It’s possible that the drinks are only dangerous when consumed a certain way by vulnerable people, said Church, who has energy drinks occasionally when he needs to pull night shifts at the hospital.

“You need to know what you’re putting in your body,” he said. “It’s just about being smart.”

Nestle, of NYU, said the problem is we really don’t know whether they are dangerous or not, because they haven’t been studied.

Still, many people are sticking with their energy drink habits.

Seth Hall, 23, of Smithfield, R.I., said he’s not worried about the safety of Monster Energy Drinks, which he’s consumed daily since age 11.

The community college student said he likes Monster’s sweetness and the extra edge he gets — especially around midterms and finals time.

“It doesn’t really work,” to improve his grades, he said, “but I try anyway.”

Although he’s cut down to just two cans a day, he would happily go back to his six-a-day habit — if, at $2.50 per can, he could afford it.