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copied content from Caffeine dependence; see that page's history for attribution

Caffeine is a commonplace central nervous system stimulant drug which occurs in nature as part of the coffee, tea, yerba mate and other plants. It is also an additive in many consumer products, most notably beverages advertised as energy drinks and colas.

Caffeine's mechanism of action is somewhat different from that of cocaine and the substituted amphetamines; caffeine blocks adenosine receptors A and A2A. Adenosine is a by-product of cellular activity, and stimulation of adenosine receptors produces feelings of tiredness and the need to sleep. Caffeine's ability to block these receptors means the levels of the body's natural stimulants, dopamine and norepinephrine, continue at higher levels.

The Diagnostic Manual for Mental Disorders describes four caffeine-related disorders including intoxication, withdrawal, anxiety, and sleep.

Dependence
Mild physical dependence can result from excessive caffeine intake. Caffeine addiction, or a pathological and compulsive form of use, has not been documented in humans.

Studies have demonstrated that people who take in a minimum of 100 mg of caffeine per day (about the amount in one cup of coffee) can acquire a physical dependence that would trigger withdrawal symptoms that include headaches, muscle pain and stiffness, lethargy, nausea, vomiting, depressed mood, and marked irritability. Professor Roland Griffiths, a professor of neurology at Johns Hopkins in Baltimore strongly believes that caffeine withdrawal should be classified as a psychological disorder. His research suggested that withdrawals began within 12–24 hours after stopping caffeine intake and could last as long as nine days. Continued exposure to caffeine will lead the body to create more adenosine receptors in the central nervous system which makes it more sensitive to the effects of adenosine in two ways. Firstly, it will reduce the stimulatory effects of caffeine by increasing tolerance. Secondly, it will increase the withdrawal symptoms of caffeine as the body will be more sensitive to the effects of adenosine once caffeine intake stops. Caffeine tolerance develops very quickly. Tolerance to the sleep disruption effects of caffeine were seen after consumption of 400 mg of caffeine 3 times a day for 7 days, whereas complete tolerance was observed after consumption of 300 mg taken 3 times a day for 18 days.