People do not view caffeine as a drug, however caffeine is probably the most commonly used drug in the world. Caffeine can be found in many products like tea, soft drinks, and energy drinks, but coffee is the source of caffeine. Caffeine can cause a mild physical dependence and some people depend on caffeine to get through their day. Caffeine withdrawal can make for a few bad days. Some symptoms caused by the lack of caffeine include headaches, constipation, depression or sleepiness. The difference between caffeine and other drugs is caffeine is legal. Caffeine does not have an intoxicating effect on a person. People do not have behavioral changes, or lose of motor skills, while people who use cocaine or methamphetamines might experience.