There are more than 500 energy drink products on the market, and their increased popularity is matched by a significant rise in energy drink-associated emergency department visits and deaths.

Manufacturers and fans of these products claim they are as safe as caffeine, but there is little evidence to support that claim.

Caffeine in doses up to 400 mg (about five cups of coffee) is generally recognized as safe by the Food and Drug Administration. While energy drinks usually contain caffeine, little is known about the safety of some of their other ingredients the study team writes in the Journal of the American Heart Association.

To see what effects these other components have, researchers compared physical changes in a group of 18 healthy men and women after consuming a commercially available energy drink and after drinking another concoction with the same amount of caffeine but none of the other ingredients.

Besides 320 mg of caffeine – the amount in about four cups of coffee – the energy drink contained 4 ounces of sugar, several B vitamins and a proprietary “energy blend” of taurine and other ingredients that are often found drinks like Monster Energy, Red Bull and 5-Hour Energy.

Sachin A. Shah of David Grant Medical Center on Travis Air Force Base and University of the Pacific in Stockton, California, and colleagues measured the participants’ blood pressure and used an electrocardiogram (often called an ECG or EKG) to measure heart electrical activity for 24 hours after the subjects consumed the drinks.

Related: FDA Checks Into Deaths Linked to Energy Drink

An ECG change known as QTc prolongation and sometimes associated with life-threatening irregularities in the heartbeat was seen after drinking the energy drink, but not after drinking the caffeine beverage, the study team reports.

Several drugs have been withdrawn from the market just for causing ECG changes of a similar magnitude, the authors note.

Blood pressure increased by close to 5 points after drinking the energy drink, but by just under 1 point after drinking the caffeine beverage. Blood pressure also remained elevated six hours later.

These changes are by no means worrisome for healthy individuals, the researchers say, but patients with certain heart conditions might need to exercise caution consuming energy drinks.

Related: Can Caffeine Kill You?

Larger studies are needed to evaluate the safety of the noncaffeine ingredients contained in energy drinks, they conclude.

“The energy drink industry claims that their products are safe because they have no more caffeine than a premium coffee house coffee,” said Dr. Jennifer L. Harris from University of Connecticut’s Rudd Center for Food Policy and Obesity in Storrs, who wasn’t involved in the study.

“However, energy drinks also contain a proprietary ‘energy blend,’ which typically consists of stimulants and other additives. Some of these ingredients (including taurine and guarana) have not been FDA-approved as safe in the food supply, and few studies have tested the effects of caffeine consumption together with these ‘novelty’ ingredients,” she said by email.

“On top of that, energy drinks are highly marketed to adolescent boys in ways that encourage risky behavior, including rapid and excessive consumption,” she said. “As a result, emergency room visits by young people in connection with energy drinks are rising.”

Any research that compares the effects of consuming energy drinks versus caffeine alone provides important evidence for public health advocates who have urged the energy drink companies to stop targeting youth with these potentially harmful products, Harris added.

Related: Energy Drinks Can Be Harmful to Young Children

More than 5,000 cases of people who got sick from energy drinks were reported to U.S. poison control centers between 2010 and 2013, and almost half of those cases were in children did not realize what they were drinking

Energy drinks typically contain high levels of sugar and at least as much caffeine as a cup of coffee. But the drinks also often tout the energy-boosting effects of a mix of other ingredients, ranging from taurine and l-carnitine, naturally occurring amino acids, to ginseng (a Chinese herb typically used in alternative medicine). But despite this “special blend” of ingredients, studies suggest energy drinks don’t boost attention any better than a cup of coffee does.

Even just one 16-ounce energy drink can increase blood pressure and stress hormones and could put a healthy young adult at risk for heart damage, concludes a 2015 Mayo Clinic study.

The American Academy of Pediatrics says energy drinks have “no place” in the diet of children and adolescents.