How Caffeine Works


It's as much a part of the morning ritual as brushing your teeth and making the bed. It gives energy drinks their zip. According to its adherents, it can alternately keep you calm, sharpen your mind or provide the vital boost to make it through an all-nighter.

Crave or avoid it, caffeine is a powerful influence in our lives.


Around 90 percent of Americans consume caffeine every single day in one form or another. More than half of all American adults consume more than 300 milligrams of caffeine every day, making it America's most popular drug by far [source: Johns Hopkins].

But the U.S. is far from the lead when it comes to national caffeine consumption. According to a 2010 report by commodities analysts for Businessweek, Scandinavian nations such as Finland consume more caffeine per capita -- mostly in coffee -- than any other country. The report noted other surprising trends, like a move in Brazil to offer coffee drinks as part of grade-school lunches [source: Wallace].

Although Americans aren't the world's biggest per-capita caffeine fiends, we're not exactly teetotalers. Research by the U.S. Food and Drug Administration (FDA) and the American Medical Association (AMA) has led these groups to consider 300 milligrams (about two cups of coffee) the upper limit of a moderate daily dose. But roughly 20 percent to 30 percent of Americans consume more than 600 milligrams -- considered a high dose of the drug -- on a typical day [source: Kovacs]. If you consume more than four cups of coffee a day, you're probably among that number.

Caffeine is a natural component of chocolate, coffee and tea, and is added to colas and energy drinks. The international medical community recognizes caffeine withdrawal as a medical syndrome, yet it's a common ingredient in diet pills and some over-the-counter pain relievers and medicines, and it's being studied for its potential benefits in battling Parkinson's disease, Alzheimer's and even cancer [source: Johns Hopkins]. Read on to learn more about this powerful drug and our complex relationship with it.

What is Caffeine?

You're not imagining it: Coffee gives your brain and metabolism a boost.
You're not imagining it: Coffee gives your brain and metabolism a boost.

Caffeine is a naturally occurring chemical stimulant called trimethylxanthine. Its chemical formula is C8H10N4O2 (see Erowid: Caffeine Chemistry for an image of the molecular structure). It is a drug, and actually shares a number of traits with more notorious drugs such as amphetamines, cocaine and heroin. As we'll explain in more detail in the next few pages, caffeine uses the same biochemical mechanisms as these other drugs to stimulate brain function: If you feel like your mind is racing after drinking one too many espressos, you're not imagining things.

In its pure form, caffeine is a white crystalline powder that tastes very bitter. It is medically useful to stimulate the heart and also serves as a mild diuretic, increasing urine production to flush fluid out of the body.


Caffeine has been an integral part of global culture for hundreds of years. African folklore sets the discovery of coffee's energizing properties around 800 A.D., European and Asian accounts indicate that coffee and tea were local staples as early as the 1400s. Although coffee was often seen as a rare luxury for societies far removed from coffee-growing regions, foods and drinks made from other caffeine-containing plants were likely part of humankind's medical and nutritional arsenal since before recorded history [source: Fredholm].

Today, caffeine is used much as it has been for generations: It provides a "boost of energy" or a feeling of heightened alertness. Many former students can recall using strong coffee or caffeine pills to stay awake while cramming for finals. Likewise, drivers on long road trips often fill their cup holders with energy drinks or convenience-store coffees to help them push through to their destinations.

Remember, though, that caffeine shares some traits of those much harder drugs -- including the ability to cause addiction. Many people feel as though they cannot function in the morning without a cup of coffee (and its caffeine-powered boost) to kick-start the day. Caffeine's effects may be much milder than those of illicit drugs, but kicking a caffeine habit can be difficult for someone who has made the drug a large part of his or her diet and lifestyle.

Caffeine is unlike many other drugs in that it is abundant in what we eat and drink. Read on to learn more about what foods provide most of the world's caffeine, and discover the many ways in which consuming caffeine has become part of global culture.

Caffeine in the Diet

Caffeine occurs in or is added to common foods and beverages such as coffee and colas. In many cultures that prohibit alcohol consumption for some or all people, caffeine is considered an acceptable social drug.
Caffeine occurs in or is added to common foods and beverages such as coffee and colas. In many cultures that prohibit alcohol consumption for some or all people, caffeine is considered an acceptable social drug.

Caffeine occurs naturally in many plants, including coffee beans, tea leaves and cocoa beans, so it’s found in a wide range of food products. Caffeine is also added to many other food products, including a variety of beverages. Coca-Cola, for example, was originally made with kola nut extract, which naturally contains caffeine and was the main source of the flavor and buzz that early fans of the beverage craved (although the cocaine contained in the drink's early formulas certainly helped increase that craving).

Colas are now made with artificial flavors, and caffeine is often added during the production process. Typical caffeinated sodas (Coke, Pepsi, Mountain Dew, Dr. Pepper, etc.) contain 35 to 55 milligrams per 12-ounce (355 milliliter) can. Products like Vault and Jolt budge up against the FDA's official limit for how much caffeine a product marketed as a soda can contain: 71 milligrams per 12-ounce (355 milliliter) can.


Energy drinks, which mix large concentrations of caffeine with sugar and other stimulants, are a relatively new trend in caffeinated beverages. They get around the FDA's limit by not calling themselves sodas: Popular drinks like Red Bull and Rockstar contain about 80 milligrams of caffeine per 8.3-ounce (245-milliliter) and 8-ounces (236-milliliter) serving, respectively.

Caffeine also appears in many popular foods. Many people think of chocolate as a caffeine-filled food, but the amount of caffeine in a chocolate bar actually varies by a wide degree, depending on the bar's blend of cocoa butter, cocoa solids, sugar, flavorings and fillers. There could be anywhere from 3 to 63 milligrams of caffeine in a 50-gram bar of chocolate. Because chocolate milk and hot cocoa are mostly milk or water, they contain far less caffeine on average -- less than 8 milligrams per 5-ounce (150-milliliter) serving.

And then there are teas and coffee, the brewed beverages with which caffeine is so closely associated. While the brewing processes and types of coffee beans or tea leaves used to produce a serving of these drinks can affect their caffeine concentration, both have the potential to contain more caffeine than even strong energy drinks. A 5-ounce (147-milliliter) serving of coffee, for example, could contain up to 150 milligrams of caffeine, while the same serving of black tea could contain as much as 80 milligrams [source: Center for Science in the Public Interest]. To put these serving sizes in perspective, if you are buying your coffee at Starbucks or a convenience store or drinking it at home or the office out of a mug, you may be consuming it in 12-, 14- or 20-ounce containers. You can calculate your approximate dose of caffeine based on your normal serving size.

Up next, we'll look at some of the medicinal uses for caffeine.

Medicinal Uses for Caffeine

Caffeine can be found in many weight loss pills to boost the metabolism. But there are a number of additional situations where caffeine can serve important medical uses.

Caffeine that has been citrated -- treated with a citrate of potassium or sodium -- can help breathing in premature babies. Young children who suffer breathing problems after surgery may also benefit from medical treatment with caffeine [source: Kovacs].


For adults, caffeine is sometimes used to kick-start other medications. It improves the effectiveness of aspirin or acetaminophen, and is used with a drug called ergotamine to treat cluster and migraine headaches. Research suggests that caffeine's ability to cause blood vessels to constrict blood flow -- as we'll discuss in the next section -- may play a role in causing these effects.

There have been more than 19,000 studies on caffeine and coffee in the past 30 years, most of which have aimed to uncover the drug's exact effects on the human body. One of the most thorough and exhaustive studies was done by Harvard University, involving 126,000 people over an 18-year period.

The findings from the Harvard study may seem surprising: They indicate that people who drink one to three cups of coffee a day are up to 9 percent less likely to contract diabetes than those who don't. For subjects who drank six or more cups of coffee per day, men slashed their chances of contracting diabetes by 54 percent, and women by 30 percent [source: Kirchheimer].

We'll explore more of the research into caffeine's benefits in a bit, but for now it's important to note that most Americans consume the bulk of their caffeine -- intentionally or not -- as a form of self-medication. Grabbing a cup of coffee when they need a pick-me-up, for example, or sipping an energy drink to keep going at the end of a long day puts many drinkers into the higher levels of FDA-recommended daily limits of the drug. Given the way this kind of consumption can affect the body, caffeine is an important drug to understand the workings of. Read more about how caffeine affects the body and brain on the next page.

Caffeine and Adenosine

Caffeine wakes you up by fooling adenosine receptors. Adenosine slows down nerve cell activity along neural pathways like these, but caffeine (which binds to the same receptors) speeds activity up.
Caffeine wakes you up by fooling adenosine receptors. Adenosine slows down nerve cell activity along neural pathways like these, but caffeine (which binds to the same receptors) speeds activity up.

Why do so many people consume so much caffeine? Why does caffeine wake you up? In short, it's all about two words: brain chemistry.

In the article How Sleep Works, the action of adenosine is discussed in detail. But while it sounds like advanced science, it's really pretty simple. As adenosine is created in the brain, it binds to adenosine receptors. This binding causes drowsiness by slowing down nerve cell activity. In the brain, this also causes blood vessels to dilate, most likely to let more oxygen into that organ during sleep.


To a nerve cell, caffeine looks like adenosine: Caffeine binds to the adenosine receptor. However, caffeine doesn't slow down the cell's activity like adenosine would. As a result, the cell can no longer identify adenosine because caffeine is taking up all the receptors that adenosine would normally bind to. Instead of slowing down because of the adenosine's effect, the nerve cells speed up. Caffeine also causes the brain's blood vessels to constrict, because it blocks adenosine's ability to open them up. This effect is why some headache medicines like Anacin contain caffeine -- constricting blood vessels in the brain can help stop a vascular headache.

Caffeine's effect on the brain causes increased neuron firing. The pituitary gland senses this activity and thinks some sort of emergency must be occurring, so it releases hormones that tell the adrenal glands to produce adrenaline (epinephrine). Adrenaline is the "fight or flight" hormone, and it has a number of effects on your body:

  • Your pupils dilate.
  • The airway opens up (this is why people suffering from severe asthma attacks are sometimes injected with epinephrine).
  • Your heart beats faster.
  • Blood vessels on the surface constrict to slow blood flow from cuts and increase blood flow to muscles.
  • Blood pressure rises.
  • Blood flow to the stomach slows.
  • The liver releases sugar into the bloodstream for extra energy.
  • Muscles tighten up, ready for action.

This explains why, after consuming a big cup of coffee, your hands get cold, your muscles grow tense, you feel excited and your heart beats faster.

Adenosine isn't the only neurotransmitter affected by caffeine. Read on to learn about how the drug affects dopamine, another important chemical in the body.

Caffeine and Dopamine

The dopamine-affecting caffeine in chocolate is part of why you feel happy when you eat it.
The dopamine-affecting caffeine in chocolate is part of why you feel happy when you eat it.

Dopamine is a neurotransmitter that activates pleasure centers in certain parts of the brain. Heroin and cocaine manipulate dopamine levels by slowing down the rate of dopamine reabsorption. Caffeine increases dopamine levels in the same way. Its effect is much weaker than heroin's, but the mechanism is the same. Researchers suspect that this dopamine connection is what contributes to caffeine addiction.

You can see why your body might like caffeine in the short term, especially if you are low on sleep and need to remain active. Caffeine blocks adenosine reception so you feel alert. It injects adrenaline into the system to give you a boost. And it manipulates dopamine production to make you feel good.


But caffeine can cause a vicious cycle of problems in the long term. For example, once caffeine-induced adrenaline wears off, you face fatigue and depression. Another cup of coffee or energy drink can get the adrenaline flowing again, but having your body in a state of emergency, jumpy and irritable all day long, isn't very healthy.

The most important long-term problem with caffeine is its effect on your sleep. The half-life of caffeine in your body is about six hours. That means that drinking a big cup of coffee containing 200 milligrams of caffeine at 3:00 p.m. will leave about 100 milligrams of that caffeine in your system at 9:00 p.m. Adenosine reception, which is affected by caffeine, is important to sleep, and especially to deep sleep. You may be able to fall asleep hours after that big cup of coffee, but your body will probably miss out on the benefits of deep sleep.

That sleep deficit adds up fast. The next day you feel worse, so you need caffeine as soon as you get out of bed. The cycle continues day after day. Once you get into this cycle, you have to keep consuming the drug to put off an inevitable comedown. Trying to quit can leave you tired and depressed, fighting splitting headaches as blood vessels in the brain dilate. These negative effects can be enough to force caffeine addicts back onto the drug [source: Johns Hopkins].

But caffeine is not a one-sided drug. Read on to learn about some ways that caffeine can actually improve your health.

Health Benefits of Caffeine

Though caffeine can be useful, be mindful that it is a drug -- and be aware of how much fat and sugar your favorite forms of it contain.
Though caffeine can be useful, be mindful that it is a drug -- and be aware of how much fat and sugar your favorite forms of it contain.

Caffeine had long been on the list of don'ts for people hoping to lead a healthy lifestyle. Doctors pointed to caffeine's negative effects on the nervous system and its track record of increasing anxiety, stress and food cravings, as well as its damaging effects on sleep quality. Recent studies, however, suggest that coffee and caffeine may actually offer some significant medical benefits.

Remember the more than 19,000 studies mentioned earlier? Those studies have uncovered a range of positive effects that caffeine seems to have on the human body:


  • Regular coffee drinkers were 80 percent less likely to develop Parkinson's disease.
  • Two cups a day reduced subjects' risk for colon cancer by 20 percent.
  • Two cups a day caused an 80 percent drop in the odds of developing cirrhosis.
  • Two cups a day cut the risk of developing gallstones in half.

Studies have also suggested that caffeine is beneficial in treating asthma, stopping headaches, boosting mood and even preventing cavities [source: Kirchheimer].

Some of these findings may have something to do with other healthful properties of the coffee bean, but most can be linked to caffeine directly. Researchers are even developing drugs for Parkinson's disease containing caffeine derivatives.

More research is uncovering potential benefits from this commonly consumed drug. A study by the Byrd Alzheimer's Institute in Tampa, Fla., showed that lab mice injected with caffeine were protected against developing Alzheimer's disease. The injections even helped reduce symptoms in those that had the disease. The findings lead doctors to believe that up to five cups of coffee a day could have the same positive effect on humans [source: Arendash].

And a 2007 study at Rutgers University suggested that regular exercise combined with daily doses of caffeine could increase the destruction of precancerous skin cells in mice. Once again, the findings have not yet been tested on humans, but the indication is that it will have similar effects [source: Lu].

Despite these recent findings, most doctors still recommend moderation in regard to caffeine intake. These studies give hope to those who stand by the value of their morning cup of Joe, but there's still a long way to go to determine the long-term effects of caffeine use.

Lots More Information

Related Articles

More Great Links

  • Arendash, G.W., et al. "Caffeine Protects Alzheimer's Mice Against Cognitive Impairment and Reduces Brain Beta-Amyloid Production," Neuroscience. Vol. 142. Page 941-52. 2006.
  • Center for Science in the Public Interest. "Caffeine Content of Food and Drugs." (Oct. 7, 2011)
  • Chudler, Eric. "Neuroscience for Kids: Caffeine." Oct. 1, 2011 (Oct. 2, 2011)
  • Dance, Rosalie A. and Sandefur, James T. "Reading This Could Help You Sleep: Caffeine in Your Body." Hands on Activities for Algebra at College. 1999. (Oct. 5, 2011)
  • Fredholm, B.B. "Notes on the history of caffeine use." Handbook of Experimental Pharmacology. Vol. 200. Pages 1-9. 2011.
  • Johns Hopkins University, Bayview Medical Center. "Caffeine Independence." (Oct. 5, 2011)
  • Johns Hopkins Medicine. "Caffeine withdrawal recognized as a disorder." Sept. 29, 2004 (Oct. 2, 2011)
  • Kirchheimer, Sid. "Coffee: The new health food?" January 26, 2004. (Oct. 5, 2011)
  • Kovacs, Betty. "Caffeine." 2011 (Oct 2, 2011)
  • Lu, Y.P., et al. "Voluntary exercise together with oral caffeine markedly stimulates UVB light-induced apoptosis and decreases tissue fat in SKH-1 mice." Proceedings of the National Academy of Sciences of the United States of America (PNAS). Vol. 104, no. 31. Page 12936-41. July 31, 2007. (Oct. 5, 2011)
  • Mayo Clinic Staff. "Caffeine: How much is too much?" March 9, 2011 (Oct. 2, 2011)
  • Wallace, Benjamin. "The World's Most Caffeinated Country." Bloomberg Businessweek. April 29, 2010 (Oct. 6, 2011)