Caffeine is mildly addictive, one possible reason that makers of soft drinks add it to their products. Many coffee drinkers experience withdrawal symptoms, such as headaches, irritability, sleepiness, and lethargy, when they stop drinking coffee.
Caffeine also keeps many people from sleeping, causes jitteriness, and affects calcium metabolism.
The caffeine in a standard cup or two of coffee is harmless to most people. But if you drink more than a couple of cups of coffee or cans of caffeine-containing soda per day or experience symptoms noted above, you should rethink your habit.
Some energy drinks contain far more caffeine than most soft drinks.
Center for Science in the Public Interest:Caffiene Content of Food & Drugs
