Energy drinks packed with caffeine can change the way the heart beats, researchers warn.
The team from the University of Bonn in Germany imaged the hearts of 17 people an hour after they had an energy drink.
The study showed contractions were more forceful after the drink.
The team told the annual meeting of the Radiological Society of North America that children and people with some health conditions should avoid the drinks.
Researcher Dr Jonas Dorner said: "Until now, we haven't known exactly what effect these energy drinks have on the function of the heart.
"The amount of caffeine is up to three times higher than in other caffeinated beverages like coffee or cola.
"There are many side effects known to be associated with a high intake of caffeine, including rapid heart rate, palpitations, rise in blood pressure and, in the most severe cases, seizures or sudden death."
The researchers gave the participants a drink containing 32mg per 100ml of caffeine and 400mg per 100ml of another chemical, taurine.
They showed the chamber of the heart that pumps blood around the body, the left ventricle, was contracting harder an hour after the energy drink was taken than at the start of the study.
Dr Dorner added: "We've shown that energy drink consumption has a short-term impact on cardiac contractility.
"We don't know exactly how or if this greater contractility of the heart impacts daily activities or athletic performance."
The impact on people with heart disease is also unknown.
However, the research team advises that children and people with an irregular heartbeat should avoid the drinks.
The British Soft Drinks Association already says the drinks are not for children.