"The results of a study showed that the ingestion of one, 250mL can of sugar-free Red Bull, in a sample of 30 healthy young adults, had an immediate detrimental effect on both endothelial function, and normal blood coagulation. This temporarily raised the cardiovascular risk in these individuals to a level comparable to that of an individual with established coronary artery disease."

http://www.sciencedirect.com/science/article/pii/S1443950608...