Caffeine is a non-prescription stimulant drug that is found naturally in many foods, particularly coffee (from which it derives its name), tea and the kola nut. It is also available in pill form over-the-counter, including being found mixed with aspirin as it enhances the painkilling effect. It is widely used and unregulated, with an estimated 90% of Americans using it on a daily basis. In plant life, caffeine acts as an insecticide. It was first isolated and named in 1819. It's primary effect is stimulation of the central nervous system, and acts within 45 minutes of ingestion to increase alertness and decrease drowsiness. It acts by passing the blood-brain barrier and binding to sites that usually accept adenosine. It is processed in the liver, which creates by-products that also increase glycerol and fatty acids in the plasma, dilate blood vessels, increase urine production, and relax muscles in the bronchial tubes.
Although moderate doses of caffeine will increase alertness, the body often develops a tolerance to it, requiring higher doses to get the same effect. Higher doses can often cause nervousness, irritability, anxiety, insomnia, headaches and palpitations. However, death from overdose is rare the lethal dose is the equivalent of 80 cups of coffee within a very short period of time.