Sugary, caffeine-infused energy drinks have become increasingly popular over recent years, largely thanks to their association with extreme sports. However, as the list of health risks linked to these stimulant soups continues to grow, a new study in the Journal of the American Heart Association has shown that consuming a single energy drink can throw your heartbeat totally out of whack.

Calls for these beverages to be banned or restricted have become louder after several deaths were linked to energy drinks. One particular brand was even removed from sale because it caused men who drank it to experience dangerously prolonged erections. Yet it is their effect on the ticker, rather than the pecker, that has always caused the most concern.

To investigate the extent of this danger, researchers gave 34 people 0.9 liters (32 ounces) of one of two commercial energy drinks or a placebo. They then used an electrocardiogram to record the electrical activity of participants’ hearts, taking a reading every 30 minutes for the next four hours.

They found that getting buzzed on an energy drink caused an increase in the heart’s QT interval, which refers to the period of time it takes the heart ventricles to electrically reset in between beats. On average, the QT interval of those who had ingested the energy drinks was between 6 and 7.7 milliseconds higher than those who had drunk a placebo, four hours later.

An irregular QT interval can be extremely dangerous and can lead to arrhythmia or other potentially fatal heart conditions. However, while these results suggest that energy drinks can disrupt the functioning of the heart for several hours, the study authors are at a loss to explain why this is the case.

Both of the drinks used in the study contained between 304 and 320 milligrams of caffeine per 32 fluid ounces, yet previous research has shown that doses below 400 don’t tend to cause any electrocardiographic changes.

The beverages also contained ingredients such as taurine and B vitamins. Taurine is an amino acid that occurs naturally within the body and has actually been found to have anti-arrhythmic properties, while B vitamins are also considered to be safe.

What is unknown, though, is how these various ingredients affect the body when ingested in combination, and whether this has something to do with the results of this research.

Summing up the pressing importance of unraveling this conundrum, study co-author Sachin Shah said in a statement that “we urgently need to investigate the particular ingredient or combination of ingredients in different types of energy drinks that might explain the findings seen in our clinical trial.”