Energy drinks high in caffeine and taurine may make the heart beat harder even an hour after consuming the drink, a new study shows. People with certain medical conditions and children should likely avoid such drinks in light of the new data, researchers warned.

Researchers from the University of Bonn in Germany gave 18 healthy participants with an average age of 27.5 an energy drink that contained 32 mg/100 mL of caffeine and 400 mg/100 mL of taurine. Taurine is a chemical that some studies suggest may improve athletic performance, though this is controversial. Researchers then used cardiac MRI scans to observe the function of the heart. The found that the contractility of the left ventricle, which is responsible for pumping blood from the heart to the rest of the body, was increased an hour after subjects consumed the drink – in other words, the heart was pumping noticeably harder.

According to the researchers, the amount of caffeine in typical energy drinks is up to three times higher than that found in other caffeinated beverages like coffee. Side effects of high caffeine consumption include rises in blood pressure, rapid heart rate or even seizures or death in very severe cases. The researchers in this study did not observe changes in participants’ heart rate or blood pressure following energy drink consumption.

The study is ongoing and the long-term effects of energy drink consumption are not known. Nevertheless, the researchers cautioned that children and people who suffer from arrhythmias should avoid energy drinks, as increasing the contractility of the heart may trigger dangerous irregular heart rhythms.

A report released this year from the Substance Abuse and Mental Health Services Administration showed that the number of emergency room visits related to energy drink consumption doubled. Most of these cases were in adults aged 18 to 39, but people over 40 also showed a 279% increase in emergency room visits related to the drinks.