Chemically speaking, pure caffeine is a plant-based alkaloid which stimulates the central nervous system of any creature that ingests it. In nature, it serves as a form of pest control for certain plants such as cacao trees, coffee shrubs, yerba 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, while tea leaves and yerba 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 is actually the scientific definition of bitter, which is why many beverages containing caffeine also contain copious amounts of sugar or other sweeteners. That 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 are caused by a central nervous reaction. The heart rate increases, blood vessels constrict, and air passages relax. 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. Without the stimulation provided by this alkaloid, the blood vessels open, increasing the flow of blood. The traditional cure for withdrawal is to ingest more caffeine. This is also why many headache medications contain small amounts.
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 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 the drug.