Diarrhea George Beinhorn mentioned cramps, and Ed Furtaw mentioned diarrhea after using these drinks a lot in an ultra. There are three contributing factors in using soda as a sports drink. Sucrose is split into glucose and fructose in the digestive tract.
Many people get diarrhea from large amounts of fructose.
As Ed notes, it tends to pull water into the intestines. That reverses the hydration process and causes diarrhea. If you take a lot of caffeinated soda, you may over dose on caffeine, particularly if you don't use it much away from the run. Combine that with the tendency for diarrhea caused by too much fructose, and you're primed for extra potty breaks. The electrolyte balance of most sodas is terrible for running.