Caffeine is natural plant alkaloid found in the coffee bean, the tea leaf, the kola nut, and the cacao podin. Coffee, tea, cocoa, chocolate, soft drinks, and some drugs all contain caffeine, and when consumed, it behaves as a central nervous system stimulant. Pure caffeine is odorless and has a bitter taste.
In moderate doses, caffeine:
- Increases alertness
- Reduces fine motor coordination
- May cause insomnia
- May cause anxiety and dizziness
- Potentiates the action of epinephrine causing increased heart rate and blood pressure
- Stimulates Dopamine secretion
- Enhances gastric acid secretion
- Promotes diuresis and may contribute to dehydration
Caffeine enters the bloodstream through absorption in the stomach and small intestine and, like other psychoactive compounds (e.g nicotine and ethanol), easily crosses the blood brain barrier. Caffeine's main biochemical affect is competitive inhibition of adenosine receptors (antagonist action). Adenosine, which shares a similar chemical structure with caffeine, in the central nervous system is important for promoting sleep. Effects may be seen within 15 minutes of consumption. Once in the body, caffeine will stay around for hours: it takes about six hours for one half of the caffeine to be eliminated.
Some studies show that caffeine causes physical dependence (addiction). One way to tell if someone "needs" that cup of coffee or bottle of Mountain Dew is to take it away from them and then see if they have any withdrawal symptoms. Typical withdrawal symptoms associated with caffeine are the headaches, fatigue, and muscle pains. These symptoms can occur within 24 hours after the last dose of caffeine. One study has stated that the minimum consumption of caffeine for physical dependence is 4 cups of coffee per day. Other studies say that a few more cups of coffee are needed to develop dependence.
Excessive caffeine consumption over time can lead to kidney stones.
Caffeine can be found in many drinks, foods, and drugs. Americans consume about 45,000,000 pounds of caffeine each year. In the United States, coffee drinkers drink an average of 2.6 cups per day. Total caffeine intake for coffee drinkers was 363.5 mg per day - this includes caffeine from coffee and other sources like soft drinks, food and drugs. Non-coffee drinkers also consume plenty of caffeine: former coffee drinkers get about 107 mg per day and people who have never had coffee get about 91 mg per day. 
- Schreiber et al., Measurement of coffee and caffeine intake: Implications for epidemiological research, Preventive Medicine, 17:280-294, 1988 and Chou, T., Wake up and smell the coffee. Caffeine, coffee and the medical consequences, West. J. Med., 157:544-553, 1992