Commercial energy drinks have once again been linked to heart health concerns—this time by a study published today in the Journal of the American Heart Association.

And it’s not the fault of caffeine alone, the study results suggest.

Previous research has associated the mix of ingredients in some popular energy beverages with a host of heart issues. The energy drinks often contain high levels of caffeine as well as blends of vitamins, herbs, and other stimulants.

Reported heart problems have included elevated blood pressure, increases in the stress hormone norepinephrine, heart palpitations, and abnormal heart rhythms, in general, even in people with no cardiovascular risk factors.

The new research adds to the growing body of evidence that these drinks can be dangerous and send some to the emergency room.

“For the 1 in 3 Americans who already have hypertension, this increase in blood pressure from consuming energy drinks could pose a potentially serious risk,” says Emily A. Fletcher, Pharm.D., study author and deputy pharmacy flight commander at David Grant U.S.A.F. Medical Center at Travis Air Force Base in California, where the study was performed.

The study compared a caffeinated drink mixture to a commercial energy drink that also contains caffeine. The results suggest that caffeine alone in these drinks may not be responsible for the heart health problems.

Here’s what you need to know about energy drinks and your heart.

Beyond Caffeine: What the Study Found

Study subjects included 18 healthy men and women ages 18 to 40. Half were given a 32-ounce serving of an unspecified commercially available energy drink. The researchers would not reveal the name of the energy drink brand.

The others received a control drink: 32 ounces of carbonated water, cherry syrup, lime juice, and caffeine.

Both drinks contained 320 milligrams of caffeine, but the energy drink also contained what the manufacturer calls its “proprietary energy blend,” a mix that includes B vitamins, amino acids taurine and L-carnitine, the sugar alcohol inositol, and plant compounds such as panax ginseng extract and guarana extract.

Researchers measured the electrical activity of the subjects’ hearts and their systolic blood pressure (the top number) immediately before consumption of the drink, and 1, 2, 4, 6, and 24 hours after.

Two hours after drinking, the hearts of the study participants who’d consumed the energy drink showed temporary abnormal electrical activity—which could raise risks of a potentially life-threatening arrhythmia. The control group did not experience the effects.  

Both groups showed an increase in systolic blood pressure for 4 hours after drinking. But those in the energy drink group still had elevated blood pressure levels 6 hours after finishing their beverage, while the blood pressure of those in the control group had returned to normal.

Fletcher, the study author, suspects that the “proprietary blend” of ingredients in the commercial energy drink may prolong caffeine’s activity in the body, prevent it from being excreted, or that these substances “may have activity of their own above and beyond caffeine.”

The American Beverage Association (ABA), the trade group representing energy drink manufacturers, challenged the study, and said, “It is also important to note that the raised blood pressure reported in this study was minimal—similar to what is experienced when climbing a flight of stairs.”

What We Still Need to Know

More research is needed on energy drinks and their ingredients to fully understand the mechanisms at work, say experts.

We already know that “energy drinks contain other ingredients that act as stimulants, such as taurine, guarana, and ginseng,” says Anna Svatikova, M.D., Ph.D., a cardiologist at Mayo Clinic who has researched energy drinks. “But not all of the ingredients have been well-studied in terms of cardiovascular effects.”

Consumers are somewhat in the dark about what’s really in energy drinks because some fail to list all of the ingredients—and their amounts—on labels, she says.

That can include caffeine. Decades of research has found that less than 400 milligrams of caffeine (the amount in about four 8-ounce cups of coffee) a day is generally safe, but it can be easy to overdo without realizing.

“Energy drink manufacturers—just like all other manufacturers—are not required by the Food and Drug Administration to disclose the amount of caffeine in their products,” says William Wallace, a policy analyst at Consumers Union, the policy and mobilization arm of Consumer Reports.

“We think it should be mandatory to list the amount of caffeine—and that companies should be required to prove the safety of all ingredients in food or drinks before they hit the market,” he says. “Right now, that’s not always the case.”

The ABA claims that the leading energy drink manufacturers already display total caffeine content on their packages.

Experts Urge Caution

Given the mounting research, experts suggest that consumers exercise caution.

“The cardiovascular responses to energy drinks—especially in children and adolescents, and adults with heart disease—aren’t totally known,” warns Svatikova, “but they are likely to be significant.”

So, what should you do?

The American Academy of Pediatrics’ current guidelines call for complete avoidance of energy drinks by children and adolescents, so keep these beverages away from them. Because it’s recommended that kids consume no more than 85 milligrams of caffeine daily, more than one energy drink could put them over the limit.

When it comes to adults, says Fletcher, “I would recommend only moderate consumption of them and avoidance during situations that would also increase your blood pressure and heart rate, such as while exercising or playing sports.”

And if you have high blood pressure, arrhythmia, or heart disease, skip them altogether. 

Editor’s Note: This article has been updated to include comments from the ABA, which we received after publication.