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. 
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.
- Fisone, G, Borgkvist A, Usiello A (2004): Caffeine as a psychomotor stimulant: Mechanism of Action. Cellular and Molecular Life Sciences 61:857-872
- Addicott, M. A. (2014). Caffeine Use Disorder: A Review of the Evidence and Future Implications. Curr Addict Rep, 1(3), 186-192. doi:10.1007/s40429-014-0024-9
- Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 15: Reinforcement and Addictive Disorders". In Sydor A, Brown RY. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. p. 375. ISBN 9780071481274.
Long-term caffeine use can lead to mild physical dependence. A withdrawal syndrome characterized by drowsiness, irritability, and headache typically lasts no longer than a day. True compulsive use of caffeine has not been documented.
- Studeville, George. “Caffeine Addiction Is a Mental Disorder, Doctors Say.” National Geographic. Jan. 15, 2010. http://news.nationalgeographic.com/news/2005/01/0119_050119_ngm_caffeine.html
- Juliano, L. M.; Griffiths, R. R. (2004). "A critical review of caffeine withdrawal: Empirical validation of symptoms and signs, incidence, severity, and associated features". Psychopharmacology. 176 (1): 1–29. doi:10.1007/s00213-004-2000-x. PMID 15448977.
- "Caffeine Pharmacology." News Medical. http://www.news-medical.net/health/Caffeine-Pharmacology.aspx