Chemically speaking, pure caffeine is a plant-based alkaloid which stimulates the central nervous system of any creature that ingests it. In nature, caffeine serves as a form of pest control for certain plants such as cacao trees, coffee shrubs, yuba mate and tea trees. The caffeine causes insects and other pests to collapse from the effects of over-stimulation.

Caffeine, also known as guaranine, mateine or theine depending on the source plant, is considered a psychoactive drug. However, it has not been designated a controlled substance, so its use in teas, coffees and sodas is not illegal. Coffee beans contain a significant amount of caffeine, while tea leaves and yuba mate plants contain less. Cocoa beans also contain caffeine, but they contain much higher levels of another alkaloid called theobromine.

When caffeine is removed from the source plant and reduced to its most natural state, it forms a white powder. This powdered form of caffeine is actually the scientific definition of bitter, which is why many beverages containing caffeine also contain copious amounts of sugar or other sweeteners. The caffeine contained in stimulant pills such as No-Doz is not diluted with sugar, so the pills can be very bitter indeed.

Caffeine may not be addictive in the classic sense, but the body does build up a tolerance over time. Some people find it difficult to function without at least one cup of strong coffee or tea in the morning. The stimulating effects of caffeine are caused by a central nervous reaction. The heart rate increases, blood vessels expand and the brain receives more oxygen. This effect can last up to an hour.

There is such a thing as caffeine withdrawal. Regular consumers of coffee or sodas may experience painful headaches if the body is denied caffeine. These headaches are caused by excess blood collecting in the area around the brain and sinus cavities. Without the stimulation provided by caffeine, the blood vessels shrink, restricting the flow of blood. The traditional cure for caffeine withdrawal is to ingest more caffeine. This is also why many headache medications contain small amounts of caffeine.

It would be very difficult to consume enough caffeine for a lethal overdose, although some deaths have been reported. One would have to drink several hundred cups of coffee or ingest at least 75 caffeine tablets to reach toxic levels, however. Taking several Vivarin or No-Doz stimulants to increase mental awareness should not be harmful in the long term, but there may be a sudden crash as the drug's effects wear off. So keep the all-night study sessions or long distance drives to a minimum if you don't want to experience the dark side of caffeine.