Energy drinks may give you more than a mental and physical boost — and it’s not good for you.

According to a study in the Journal of the American Heart Association, a 32-ounce energy drink with 320 milligrams of caffeine resulted in more profound changes in the heart’s electrical activity and blood pressure than a 32-ounce control drink with the same amount of caffeine.

While the U.S. Food and Drug Administration generally considers caffeine in doses of less than 400 mg safe, energy drinks often include proprietary energy blends. With more than 500 types of energy drinks on the market, energy drink-associated emergency room visits and deaths have increased, prompting questions about their safety.

“We decided to study energy drinks’ potential heart health impact because previous research has shown 75 percent of the base’s military personnel have consumed an energy drink,” said Emily A. Fletcher, Pharm.D., deputy pharmacy flight commander at the David Grant U.S.A.F. Medical Center at Travis Air Force Base in Fairfield, California. “And nearly 15 percent of military personnel, in general, drink three cans a day when deployed, which is more than we studied here.”

For the study, researchers randomly divided 18 participants into two groups. The first group drank a commercially available energy drink containing 108 grams of sugar, 320 mg of caffeine and various other compounds. The second group had a control drink containing 320 mg of caffeine, 40 milliliters of lime juice and 140 ml of cherry syrup in carbonated water.

After a six-day washout, participants switched drinks.

Researchers measured the electrical activity of participants’ hearts using an electrocardiogram. They also measured their peripheral and central blood pressures at the study’s start and at one, two, four, six and 24 hours after drink consumption.

Peripheral blood pressure is the measurement of the pressure in an outlying artery, typically an upper arm. Central blood pressure is the measurement of the pressure in the aorta near the heart.

Those in the energy drink group had a corrected QT interval 10 milliseconds higher at two hours than the caffeine group.

“The QT interval is the measurement of the time it takes ventricles in the heart (the lower chambers) to repolarize, or prepare to generate a beat again,” Fletcher said. “It’s the pause from the end of the electrical impulse generating the heart to beat to the next impulse.”

To put the 10-millisecond difference into perspective, medications that affect the corrected QT interval by 6 milliseconds have label warnings, Fletcher said.

“If this time interval, which is measured in milliseconds, is either too short or too long, it can cause the heart to beat abnormally. The resulting arrhythmia can be life-threatening,” she said.

Both groups had similar increases in systolic blood pressure, but levels in the caffeine group had almost returned to their original readings after six hours.

“On the other hand, those who consumed the energy drinks still had a mildly elevated blood pressure after six hours,” Fletcher said. “This suggests that ingredients other than caffeine may have some blood pressure-altering effects, but this needs further evaluation.”

Based on this preliminary evidence in young, healthy adults, people who have high blood pressure, underlying cardiac conditions or other health issues might want to avoid or use caution when consuming energy drinks until more is known about their impact on heart health, Fletcher said.