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Link? Caffeine is one of the most used, if not the most used and one of the most tested drugs available.
Most deaths in marathons I've read about are things like unknown heart conditions being exposed when relatively low-trained runners are overworked in marathon conditions. There was also the girl that died on Jack3d supplement that contained ephedra or DHEA or some other stimulant but it wasn't caffeine.
I know caffeine isn't currently banned but it has been in the past and it is, clearly, a performance-enhancing drug. Taking it in that way might help you stay awake but wouldn't do much for your long term health. Personally, I wouldn't take it.