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Caffeine is that little boost people need to make it through the day. But what exactly is caffeine? Caffeine, in its purest form, is a plant-based organic compound that stimulates the nervous system. Also known as guaranine, mateine, or theine, caffeine is considered a psychoactive drug. But, it hasn’t been deemed a controlled substance, so commercial use is not illegal. When taken to its purest form, caffeine becomes a white powder. This powder is the scientific definition of bitter. This is why almost all drinks that contain caffeine also have large amounts of sugar and sweeteners to counter act the bitter flavor.
Caffeine also has many health affects. When ingesting caffeine, the heart rate increases, the blood vessels will expand, and the brain will get more oxygen. Caffeine withdrawal can occur when the body is denied the dose of caffeine it has become accustom to. Caffeine withdrawal is associated with painful headaches caused by the gathering of blood collecting around the brain and sinus cavities. Without the caffeine, the blood vessels shrink and blood flow is restricted. The cure for the withdrawal is to ingest caffeine. This explains why a lot of headache medicines contain caffeine. Overdosing on caffeine is possible but very difficult. To reach toxic levels, a person would have to drink over one-hundred cups of coffee or seventy-five caffeine tablets.

Bibliography:

http://www.wisegeek.com/what-is-caffeine.htm