What is caffeine?

Quick Answer
An addictive chemical substance found in foods such as coffee, tea, chocolate, and some soft drinks.
Expert Answers
enotes eNotes educator| Certified Educator
Structure and Functions

Caffeine, known chemically as trimethylxanthine, is a substance found naturally in coffee beans, cocoa beans, and tea leaves. Caffeine renders significant physiological effects on its consumers, and individuals often use it in an effort to boost energy or wake up. In addition to its natural occurrences, caffeine is added by food processors to a variety of foods, including select soft drinks. Pills formulated to fight fatigue that are available without a prescription usually contain high doses of caffeine. A six-ounce cup of drip-brewed coffee contains around one hundred milligrams of caffeine, while an equal volume of tea contains seventy milligrams. A twelve-ounce can of cola has about fifty milligrams of caffeine, while a fifty-gram chocolate bar can contain between five and sixty milligrams.

In the brain, the chemical compound adenosine binds to adenosine receptors, which leads to drowsiness and decreased firing of neurons. Adenosine also interacts with blood vessels to dilate them. Caffeine has an appearance similar to that of adenosine, and it will bind to adenosine receptors, thus preventing adenosine from doing so. As a result, adenosine cannot initiate drowsiness, and alertness is increased. Under the influence of caffeine, blood vessels constrict and neurons fire rapidly. The brain responds to these cues by releasing the hormone adrenaline, commonly referred to as the “fight or flight” hormone. Adrenaline causes the pupils of the eyes to dilate, increases heart rate and blood pressure, constricts blood flow to the body surface and increases blood flow to the muscles, and releases sugar from the liver into the bloodstream, among other physiological effects.

Disorders and Diseases

Caffeine is the most widely used psychoactive chemical in the world. In moderate doses, caffeine can increase alertness, but it may also cause insomnia, nervousness, and decreased fine motor coordination. While alertness is perceived by the consumer, caffeine does not increase energy. Instead, the body experiences stress, which leads to the physical sensations described above.

Regular caffeine consumption can cause physical dependence. Caffeine withdrawal is associated with headaches, irritability, and drowsiness.

One of the most serious consequences of caffeine consumption is insomnia. The amount of time that it takes half of the caffeine consumed to leave the body is about six hours. It may take up to twelve hours for the body to process caffeine and permit normal sleep.

Caffeine consumption has also been associated with depletion of B vitamins, increased calcium loss and an accompanying risk of osteoporosis, and decreased ability to absorb iron from food. In addition, it has been shown to raise the blood level of the amino acid homocysteine, which is associated with an elevated risk of heart attack. For those with anxiety disorders, caffeine may exacerbate their symptoms.


A.D.A.M. Medical Encyclopedia. "Caffeine in the Diet." MedlinePlus. 5 May 2011. Web. 1 May. 2015.

"Caffeine Content for Coffee, Tea, Soda, and More." Mayo Clinic. Mayo Foundation, 13 May 2014. Web. 1 May. 2015.

"Caffeine: How Much Is Too Much?" Mayo Clinic. Mayo Foundation, 9 Mar. 2011. Web. 1 May. 2015.

Cherniske, Stephen. Caffeine Blues: Wake Up to the Hidden Dangers of America’s Number-One Drug. New York: Warner, 1998. Print.

Pohanka, Miroslav. "The Effects of Caffeine on the Cholinergic System." Mini Reviews in Medicinal Chemistry 14.6 (2014): 543–49. Print.

Smith, Barry D., Uma Gupta, and Bhupendra Gupta, eds. Caffeine and Activation Theory: Effects on Health and Behavior. Boca Raton: Taylor, 2006. Print.

Tolley, Aimee S. Caffeine: Consumption, Side Effects, and Impact on Performance and Mood. New York: Nova, 2014. Print.

Weinberg, Bennett Alan, and Bonnie K. Bealer. The World of Caffeine: The Science and Culture of the World’s Most Popular Drug. New York: Routledge, 2001. Print.

Zyla, Gail. "The Highs and Lows of Caffeine." Health Library. 26 Feb. 2013. Web. 1 May. 2015.