It may be the dangerous changes in heartbeat and blood pressure that land some consumers of energy drinks in the emergency room, based on results of a clinical trial that tested their short-term effects on the heart. Findings were recently published in the Journal of the American Heart Association and suggest that anyone with high blood pressure or heart rhythm disorders should use caution when consuming energy drinks.

Conducted at the University of the Pacific in Stockton, CA, this study explored known health concerns associated with energy drinks. Energy drinks—which contain a mixture of caffeine and other energy-boosting ingredients—have been linked to a number of health problems, including abnormal heart rhythm, heart attack, and sudden cardiac arrest. In 2011 alone, energy drinks caused more than 20,000 hospital visits and have led to at least 34 deaths to date.

However, just how energy drinks cause the increase in health risks is not fully clear. To learn more, researchers tracked heart activity in 34 healthy volunteers who consumed two popular brands of energy drinks (or a placebo sugar drink) on three separate occasions. The goal was to see how energy drinks affect blood pressure and changes in QT interval, which is the time it takes the heart to recharge in between beats.

The energy drinks tested in the study contained between 152 and 160 milligrams of caffeine per can, along with other ingredients like taurine, glucuronolactone, and vitamins. The placebo drink contained only carbonated water, lime juice, and cherry flavoring.

After tracking participants’ heart rhythm and blood pressure for 4 hours after consuming the drinks, researchers found that the two energy drinks significantly increased blood pressure compared to the placebo drink. After consuming the energy drinks, participants’ average QT interval was also 6–7 milliseconds higher than it was after consuming the placebo drink.

What this shows, according to authors, is that energy drinks likely have an immediate impact on both blood pressure and heart rhythm. These changes could be to blame for the increase in heart risks from energy drink consumption.

A long QT interval can signal a heart rhythm disorder, which can cause serious irregular heart rhythms that increase risk for stroke. High blood pressure, which affects nearly half of US adults, is also a known risk factor for heart disease and significantly increases risk for heart events.

For these reasons, experts urge individuals with heart conditions and high blood pressure to use caution and limit their energy drink intake. Authors also encourage additional research to study the long-term health effects of regular energy drink consumption and to help identify the ingredients that pose a danger to heart health.