Caffeine is a naturally occuring substance found in tea and coffee often added to colas, soft drinks, and energy drinks. Caffeine not only a stimulant,but also acts as a diuretic. Caffeine binds to Adenosine Receptors and is therefore regarded as an Adenosine Antagonist. After drinking a few cups of coffee, 50% of Adenosine Receptors are totally blocked by Caffeine.