Caffeine is a substance that helps the brain and nerves stay active. It belongs to a group of chemicals called methylxanthines and is the most widely used psychoactive substance worldwide. People use it to help them stay awake, improve physical performance, or boost brain function. It is also used in social settings. Caffeine works by blocking a chemical called adenosine in the brain, which normally makes people sleepy. This blocking action also increases the release of another brain chemical called acetylcholine. Caffeine has a shape similar to adenosine, which allows it to attach to the same receptors. It also increases levels of a substance called cyclic AMP, releases calcium from cells, and affects a brain chemical called GABA, but these effects usually happen at higher levels than those found in typical drinks.

Caffeine is a bitter, white, crystal-like substance related to the building blocks of DNA and RNA. It is found in the seeds, fruits, nuts, or leaves of plants in Africa, East Asia, and South America. These plants use caffeine to protect themselves from being eaten and to prevent nearby seeds from growing. The most common sources of caffeine for people are tea leaves from the Camellia sinensis plant and coffee beans from the Coffea plant. People drink caffeine-containing drinks like tea, coffee, and cola to stay alert or improve thinking. These drinks are made by steeping plant parts in water, a process called infusion. In 2020, nearly 10 million tons of coffee beans were consumed globally. Caffeine is the most widely used psychoactive drug in the world and is legal in most places. It is often seen as acceptable in many cultures.

Caffeine has both helpful and harmful effects on health. It can help treat breathing problems in premature babies and is listed as an essential medicine by the World Health Organization. It may also help reduce the risk of Parkinson’s disease. Caffeine can improve reaction time and accuracy for mental tasks. However, some people may have trouble sleeping or feel anxious after drinking it. During pregnancy, the effects of caffeine are not fully clear, and some experts suggest limiting it to about two cups of coffee a day. Caffeine can cause mild dependence, leading to symptoms like tiredness, headaches, or irritability when someone stops using it. Regular use can reduce the body’s sensitivity to caffeine’s effects, making it harder to feel alert.

The U.S. Food and Drug Administration (FDA) considers caffeine safe for most people. A lethal dose is over 10 grams per day, which is much higher than the typical daily intake of less than 500 milligrams. The European Food Safety Authority says up to 400 milligrams per day is safe for non-pregnant adults, and up to 200 milligrams per day is safe for pregnant or nursing women. A 6-ounce cup of coffee usually contains 50–175 milligrams of caffeine, depending on the type of bean, roasting, and preparation method. It would take about 50–100 regular cups of coffee to reach a lethal dose. However, pure powdered caffeine, sold as a supplement, can be dangerous even in small amounts.

Uses

Caffeine is used to help prevent and treat bronchopulmonary dysplasia in premature infants. It may help these babies gain weight during treatment and may lower the chances of developing cerebral palsy or delays in language and thinking skills. However, some long-term effects of caffeine use may be subtle.

Caffeine is the main treatment for apnea of prematurity, which is when a baby stops breathing for short periods. It is not used to prevent this condition. Caffeine is also used to treat orthostatic hypotension, a type of low blood pressure that happens when standing up.

Some people drink coffee or tea to try to manage asthma symptoms. However, there is not enough strong evidence to support this practice. Small amounts of caffeine may improve airflow in people with asthma, increasing a measurement called forced expiratory volume (FEV1) by 5% to 18% for up to four hours.

Adding caffeine (100–130 mg) to common pain relievers like paracetamol or ibuprofen slightly increases the number of people who feel pain relief.

Drinking caffeine after abdominal surgery may help the bowels return to normal function faster and reduce the time a person stays in the hospital.

Caffeine was once used as a second option for treating ADHD. It is less effective than medications like methylphenidate or amphetamine but more effective than a placebo for children with ADHD. People with ADHD, including children, teenagers, and adults, often consume caffeine, possibly as a way to self-treat their symptoms.

Caffeine is a stimulant that affects the central nervous system. It may help reduce tiredness and sleepiness. At normal doses, caffeine has different effects on learning and memory, but it generally improves reaction time, wakefulness, focus, and motor coordination. The amount of caffeine needed to have these effects varies based on a person’s size and how much caffeine they usually consume. Effects usually begin about one hour after drinking caffeine and last for three to four hours.

Caffeine can delay or prevent sleep and may improve performance during times of sleep deprivation. Workers who stay awake for long hours and use caffeine make fewer mistakes caused by tiredness.

Caffeine increases alertness in both tired and normal individuals, depending on the amount consumed.

A review of studies from 2014 found that combining caffeine with L-theanine may work together to improve alertness, attention, and the ability to switch between tasks. These effects are strongest in the first hour after taking the combination.

A 2025 review found that caffeine can improve reaction time and accuracy for mental tasks. Higher doses may improve reaction time further but can reduce accuracy after reaching certain levels.

People who regularly consume caffeine may develop a tolerance to its effects quickly. After a period of not drinking caffeine, they may feel slightly less tired or have better focus, but this does not make them perform better than people who do not use caffeine regularly.

Caffeine is known to improve physical performance in both aerobic (endurance) and anaerobic activities. Moderate doses (about 5 mg/kg) can help with sprint performance, cycling, running, and endurance by delaying muscle and central fatigue. Caffeine also increases the body’s metabolism and helps burn fat during aerobic exercise, especially in people with lower fitness levels.

Caffeine can improve muscle strength, power, and endurance. It may also help with anaerobic tests. Caffeine reduces the feeling of effort during constant load exercise, which can improve performance. This effect is not seen during exercises that continue until exhaustion, but performance is still better. Caffeine also improves power output and reduces time to finish aerobic tasks, especially in longer exercises.

For healthy adults, Health Canada recommends a daily caffeine intake of no more than 400 mg. This limit was found to be safe in a 2017 review of caffeine safety.

In healthy children, moderate caffeine intake (under 400 mg) usually causes only small and harmless effects. Infants as young as six months can process caffeine at the same rate as adults. However, higher doses (over 400 mg) may harm children, especially those with mental health or heart conditions. There is no evidence that coffee stops children from growing. The American Academy of Pediatrics advises against caffeine consumption in children and teens, especially from energy or sports drinks, based on a review of research from 1994 to 2011. For children under 12, Health Canada suggests a maximum of 2.5 mg/kg of body weight per day.

Health Canada has not set specific guidelines for adolescents due to limited data but recommends a daily intake of no more than 2.5 mg/kg of body weight. This is a cautious recommendation, as older or heavier adolescents may tolerate adult levels of caffeine without harm.

During pregnancy, caffeine is processed more slowly, especially in the third trimester, and its half-life can increase to 15 hours (compared to 2.5–4.5 hours in non-pregnant adults). Evidence about caffeine’s effects on pregnancy and breastfeeding is unclear. The UK Food Standards Agency advises pregnant women to limit caffeine to less than 200 mg per day, about two cups of instant coffee. The American Congress of Obstetricians and Gynecologists (ACOG) states that up to 200 mg per day is safe for pregnant women. For pregnant or breastfeeding women, Health Canada suggests a maximum of 300 mg per day, or about two 8-ounce cups of coffee. A 2017 review found no strong evidence that caffeine up to 300 mg/day harms pregnancy outcomes.

Scientific studies have conflicting results about caffeine use during pregnancy. One review found no increased risk of birth defects, miscarriage, or growth problems even with moderate to high caffeine intake. Other reviews have reached different conclusions.

Adverse effects

Caffeine in coffee and other caffeinated drinks can change how the stomach moves food and how much acid it produces. In women who have gone through menopause, drinking a lot of caffeine may speed up bone loss. Caffeine, along with other factors like stress and tiredness, can also increase pressure in muscles, such as those around the eyes.

When people who have not had caffeine for days or weeks drink a large amount (at least 250–300 mg, which is about 2–3 cups of coffee or 5–8 cups of tea), their bodies may produce more urine for a short time. This happens because caffeine causes the kidneys to release more water and salt. This effect may increase the risk of becoming dehydrated. However, people who regularly drink caffeine often become used to this effect and do not produce more urine.

Common side effects of caffeine that are not serious enough to need psychiatric help include mild anxiety, feeling jittery, trouble sleeping, taking longer to fall asleep, and difficulty with coordination. Caffeine can make anxiety disorders worse. A 2011 review of studies found that caffeine may cause or worsen anxiety and panic disorders in people with Parkinson’s disease. At high doses (more than 300–400 mg), caffeine can cause or make anxiety worse. For some people, stopping caffeine use may reduce anxiety.

In moderate amounts, caffeine has been linked to fewer symptoms of depression and a lower risk of suicide. Two studies suggest that drinking more coffee or caffeine may lower the risk of depression.

Some textbooks say caffeine makes people feel slightly happy, while others say it does not.

Caffeine-induced anxiety disorder is a type of substance-induced anxiety disorder listed in the DSM-5.

Whether caffeine can lead to addiction depends on how addiction is defined. People do not usually drink caffeine compulsively, so it is not generally considered addictive. However, some medical systems, like ICD-9 and ICD-10, include caffeine addiction in broader categories.

Caffeine does not seem to be something people strongly want to keep using. In some studies, people preferred a placebo (a fake treatment) over caffeine. Some research says caffeine does not have a strong biochemical reason for addiction, while other research says it may affect the brain’s reward system.

The ICD-9 and ICD-10 include "caffeine addiction" as a diagnosis, but this was debated because there is not enough evidence to support it. The DSM-5 does not list caffeine addiction as a diagnosis but suggests criteria for further study. As of 2021, the World Health Organization does not classify caffeine as an addictive substance.

Stopping caffeine use may cause mild to severe problems in daily life, such as trouble focusing, feeling sad or irritable, flu-like symptoms, headaches, or tiredness. About 11% of people report withdrawal symptoms, but only half of them actually experience them in lab tests. Withdrawal symptoms may happen if someone drinks more than 100 mg of caffeine daily and then stops. These symptoms usually last less than a day. Some people may also feel psychological dependence during withdrawal. To be diagnosed with caffeine withdrawal, a person must have used caffeine daily for a long time, then reduce their use by a lot, and have at least three of the listed symptoms that affect their daily life.

The ICD-11 includes caffeine dependence as a separate diagnosis, similar to the DSM-5’s proposed criteria for "caffeine-use disorder." Caffeine-use disorder means someone depends on caffeine even when it harms their health. The DSM-5 says there is enough evidence to study caffeine dependence but is unsure how serious the disorder is. Because of this, caffeine-use disorder is listed as a "condition for further study."

People may become used to caffeine’s effects, such as increased blood pressure and feelings of nervousness, but these changes are not very strong. Some positive effects, like feeling alert or happy, may become stronger with regular use. Tolerance to caffeine varies. High doses (750 to 1200 mg daily) can lead to full tolerance for some effects but not all. Lower doses (100 mg daily, like one cup of coffee or two to three servings of caffeinated soda) may still cause sleep problems. People who do not drink caffeine regularly are most likely to have trouble sleeping after caffeine. Some people develop tolerance to caffeine’s negative effects on sleep, but others do not.

There is not enough evidence to confirm that caffeine protects against Alzheimer’s disease or dementia.

Caffeine may help reduce the severity of acute mountain sickness if taken before climbing to high altitudes. One review found that caffeine use is linked to a lower risk of type 2 diabetes. Regular caffeine use may lower the risk of Parkinson’s disease and slow its progression.

Caffeine increases eye pressure in people with glaucoma but does not seem to affect people without the condition.

The DSM-5 also lists other caffeine-related disorders, such as caffeine-induced anxiety disorder, caffeine-induced sleep disorder, and unspecified caffeine-related disorders. The first two are grouped under "Anxiety Disorder" and "Sleep-Wake Disorder" because they share similar traits. Other disorders that cause significant problems but do not fit specific categories are listed under "Unspecified Caffeine-Related Disorders."

Some people say caffeine causes a drop in energy hours after drinking it, but this is not well studied.

Overdose

Consumption of 1–1.5 grams (1,000–1,500 mg) of caffeine per day is linked to a condition called caffeinism. Caffeinism often involves caffeine dependence and may cause uncomfortable symptoms such as nervousness, irritability, restlessness, trouble sleeping, headaches, and fast heartbeats after caffeine use.

Caffeine overdose can lead to a temporary condition called caffeine intoxication, which affects the central nervous system. This condition usually happens after consuming very large amounts of caffeine, much more than what is found in regular caffeinated drinks or tablets (e.g., more than 400–500 mg at once). According to the DSM-5, caffeine intoxication may be diagnosed if five or more of the following symptoms occur after recent caffeine use: restlessness, nervousness, excitement, insomnia, flushed face, increased urination, stomach discomfort, muscle twitching, rapid speech, fast heartbeat or irregular heart rhythm, extreme energy, and physical restlessness.

The International Classification of Diseases (ICD-11) notes that extremely high caffeine intake (e.g., more than 5 grams) may cause caffeine intoxication with symptoms like mania, depression, poor judgment, confusion, impulsive behavior, false beliefs, hallucinations, or severe muscle breakdown.

High caffeine consumption in energy drinks (at least one liter or 320 mg of caffeine) has been linked to short-term heart-related issues, such as high blood pressure, an extended heart rhythm interval, and heart palpitations. These effects are not typically seen with smaller amounts of caffeine in energy drinks (less than 200 mg).

As of 2007, no antidote or reversal agent for caffeine intoxication is known. Mild cases are treated by addressing symptoms, while severe cases may require dialysis or other medical procedures. Intralipid infusion therapy may be used if there is a risk of cardiac arrest to remove caffeine from the blood.

Death from caffeine ingestion is rare and usually results from intentional medication overdoses. In 2016, 3,702 caffeine-related exposures were reported to U.S. Poison Control Centers, with 846 requiring medical treatment and 16 cases resulting in serious outcomes. Some caffeine-related deaths are described in medical studies. The LD50 of caffeine in rats is 192 milligrams per kilogram of body weight. The estimated lethal dose for humans is 150–200 milligrams per kilogram, or about 10.5–14 grams for a typical 70 kg (150 lb) adult, equivalent to 75–100 cups of coffee. Some cases of death have occurred with lower doses, such as 57 milligrams per kilogram. Fatalities have also been linked to overdoses of powdered caffeine supplements, where less than a tablespoon may be lethal. The lethal dose is lower in people with impaired caffeine metabolism due to genetics or chronic liver disease. A case report from 2013 described the death of a man with liver cirrhosis who overdosed on caffeinated mints.

Interactions

Caffeine interacts with CYP1A2 and other substances through various processes.

According to DSST, alcohol reduces performance on standardized tests, while caffeine improves performance. When alcohol and caffeine are used together, caffeine’s effects change, but alcohol’s effects remain unchanged. For example, drinking more caffeine does not reduce alcohol’s impact. However, alcohol can reduce the feelings of jitteriness and alertness caused by caffeine. Alcohol alone reduces both the inhibitory and activational parts of behavioral control. Caffeine reduces the activational part of behavioral control affected by alcohol but does not affect the inhibitory part. The Dietary Guidelines for Americans advise avoiding alcohol and caffeine together, as combining them may lead to increased alcohol use and a higher risk of alcohol-related injuries.

Smoking tobacco increases caffeine clearance by 56% because chemicals in tobacco, called polycyclic aromatic hydrocarbons, increase the activity of the CYP1A2 enzyme. This enzyme helps the body process caffeine; more enzyme activity means caffeine is removed from the body faster, which is linked to higher coffee consumption among regular smokers.

Birth control pills can increase caffeine’s half-life by up to 40%, meaning caffeine stays in the body longer. This requires more attention to how much caffeine is consumed.

Caffeine can sometimes improve the effectiveness of certain medications, such as those used for headaches. Studies show caffeine can increase the strength of some over-the-counter pain relievers by 40%.

The effects of adenosine, a substance in the body, may be reduced in people who consume large amounts of methylxanthines like caffeine. Other methylxanthines include medications like theophylline and aminophylline, which are used to treat asthma or COPD.

Pharmacology

When a person is awake and alert without caffeine, very little adenosine is present in the neurons of the central nervous system (CNS). As a person stays awake for a long time, adenosine builds up in the spaces between neurons called synapses. This adenosine then binds to adenosine receptors on certain CNS neurons. When these receptors are activated, they cause a cellular response that increases the feeling of sleepiness. When caffeine is consumed, it blocks adenosine from attaching to its receptors. This prevents the receptors from being activated, temporarily reducing sleepiness and helping a person stay alert.

Caffeine acts as a blocker of adenosine A2A receptors. Studies with mice that lack these receptors show that blocking A2A receptors is linked to caffeine’s ability to promote wakefulness. Blocking A2A receptors in a brain region called the ventrolateral preoptic area (VLPO) reduces the release of a neurotransmitter called GABA, which normally inhibits another brain region called the tuberomammillary nucleus. This brain region releases histamine, which helps promote wakefulness. This process explains how caffeine increases alertness. Caffeine blocks all four types of adenosine receptors (A1, A2A, A2B, and A3), but it is most effective at blocking the A2A receptor. The strength with which caffeine binds to these receptors varies, with the highest binding strength at the A2A receptor (2.4 μM) and the lowest at the A3 receptor (80 μM).

Blocking adenosine receptors with caffeine also affects other brain areas. It stimulates regions that control breathing, heart rate, and blood vessel constriction. This leads to faster breathing, slower heart rate, and narrower blood vessels. Blocking adenosine receptors also increases the release of neurotransmitters like monoamines and acetylcholine, which contribute to caffeine’s stimulating effects. Adenosine normally suppresses activity in the CNS, but caffeine prevents this suppression. Heart palpitations can occur because caffeine blocks the A1 receptor.

Caffeine is both water-soluble and lipid-soluble, allowing it to cross the blood–brain barrier easily. Once in the brain, caffeine mainly acts by blocking adenosine receptors. The caffeine molecule is similar in structure to adenosine, so it can attach to adenosine receptors without activating them, acting as a competitive blocker.

In addition to blocking adenosine receptors, caffeine also blocks certain other receptors in the body. It blocks inositol trisphosphate receptor 1 and activates ryanodine receptors (RYR1, RYR2, and RYR3). It also blocks the ionotropic glycine receptor.

Caffeine does not directly bind to dopamine receptors, but it influences how dopamine interacts with its receptors. It does this by blocking adenosine receptors that form complexes with dopamine receptors, such as the A1–D1 receptor complex and the A2A–D2 receptor complex. The A2A–D2 receptor complex is a key target of caffeine because it helps explain some of its stimulating effects and how it interacts with other drugs that affect dopamine.

Caffeine also increases dopamine release in specific brain regions, such as the dorsal striatum and nucleus accumbens core, but not in the nucleus accumbens shell. This happens when caffeine blocks A1 receptors on dopamine neurons and A1–A2A receptor complexes on glutamate neurons. Over time, regular caffeine use leads to reduced dopamine release in the nucleus accumbens core due to the body’s adaptation. Long-term caffeine use also increases the number of A1 and A2A receptors in brain areas related to alertness and motivation, which reduces caffeine’s effectiveness over time.

Like other xanthines, caffeine also acts as a phosphodiesterase inhibitor. This means it increases levels of cyclic AMP in cells, activates protein kinase A, and reduces inflammation by inhibiting the production of TNF-alpha and leukotrienes. Caffeine also slightly inhibits the enzyme acetylcholinesterase, which is involved in the cholinergic system.

Caffeine from coffee or other drinks is absorbed by the small intestine within 45 minutes of drinking and spreads throughout the body. The highest level of caffeine in the blood is reached within 1–2 hours. The body removes caffeine through a process called first-order kinetics. Caffeine can also be absorbed through the rectum, as seen in suppositories used for migraines and hyperemesis. However, absorption through the rectum is less efficient than through the mouth, with only about 30% of the caffeine being absorbed compared to oral intake.

The time it takes for the body to eliminate half of a caffeine dose (the half-life) varies among people based on factors like pregnancy, other medications, liver function, and age. In healthy adults, the half-life is between 3 and 7 hours. Smoking decreases the half-life by 30–50%, while taking oral contraceptives doubles it. During the last trimester of pregnancy, the half-life increases significantly. In newborns, the half-life can be 80 hours or more, but it decreases rapidly as the child grows. The antidepressant fluvoxamine greatly reduces caffeine’s elimination, increasing its half-life from 4.9 hours to 56 hours.

Caffeine is broken down in the liver by the cytochrome P450 enzyme system, especially the CYP1A2 enzyme, into three main compounds:

• Paraxanthine (84%): Increases the breakdown of fats, raising levels of glycerol and free fatty acids in the blood.
• Theobromine (12%): Widens blood vessels and increases urine production. It is also found in cocoa beans.
• Theophylline (4%): Relaxes the muscles in the airways and is used to treat asthma. However, much higher doses of theophylline are needed for medical use than those achieved through caffeine metabolism.

A minor caffeine metabolite is 1,3,7-trimethyluric acid. Another is 7-methylxanthine. These metabolites are further broken down and excreted in the urine. People with severe liver disease may have higher caffeine levels in their bodies because their livers cannot break it down as efficiently.

A 2011 review found that some people have genetic variations that increase how

Chemistry

Pure anhydrous caffeine is a white, bitter-tasting, and odorless powder. It melts at a temperature between 235 and 238 °C. Caffeine dissolves moderately in water at room temperature (2 grams per 100 milliliters) but dissolves quickly in boiling water (66 grams per 100 milliliters). It also dissolves moderately in ethanol (1.5 grams per 100 milliliters). Caffeine is weakly basic, with a pK a of the conjugate acid around 0.6. This means it requires a strong acid to become protonated. Caffeine has no stereogenic centers, so it is classified as an achiral molecule.

The xanthine core of caffeine contains two fused rings: a pyrimidinedione and an imidazole. The pyrimidinedione has two amide functional groups that mostly exist in a zwitterionic resonance. In this resonance, the nitrogen atoms are double-bonded to their adjacent amide carbon atoms. As a result, all six atoms in the pyrimidinedione ring are sp hybridized and planar. The imidazole ring also has a resonance. Therefore, the fused 5,6 ring core of caffeine contains ten pi electrons, making it aromatic according to Hückel's rule.

Caffeine can gain a proton to form a positively charged molecule called caffeinium. This cationic form is the main way caffeine exists in acidic solutions.

Commercial sources

The world's supply of pure caffeine, used in drinks, medicines, and other products, comes from two sources: industrial production and removing caffeine from natural sources. Even though these methods are different, the final caffeine products are the same chemically and have the same effects on the body. Studies show that synthetic caffeine works the same way in the body as natural caffeine. Some people believe natural caffeine is absorbed more slowly and causes a less sudden drop in energy after its effects wear off, but there is not much scientific proof to support this idea. However, consumer demand for natural caffeine has grown so much that caffeine removed from natural sources is now often seen as a byproduct of caffeine production.

In 2022, the global market traded 128,127 tons of pure caffeine. Most synthetic caffeine is made by Chinese pharmaceutical companies, but exact numbers about how much caffeine comes from synthetic versus natural sources are not available.

Natural and synthetic caffeine can be told apart by looking at the ratio of carbon-13 to carbon-12 in their molecules. Most of the carbon in synthetic caffeine comes from petroleum sources, which have a different carbon isotope signature compared to natural sources.

Natural occurrence

Approximately thirty types of plants are known to contain caffeine. Common sources include the "beans" (seeds) of two coffee plants, Coffea arabica and Coffea canephora (about 1.3% caffeine is typical); the seeds of the cocoa plant, Theobroma cacao; the leaves of the tea plant; and kola nuts. Other sources are the leaves of yaupon holly, South American holly (yerba mate), and Amazonian holly (guayusa); and the seeds of Amazonian maple (guarana berries). Plants containing caffeine also grow in temperate climates around the world, though these are not related to the previously mentioned plants.

Caffeine in plants functions as a natural pesticide. It can paralyze and kill insects that feed on the plant. Coffee seedlings often have high caffeine levels when they are growing new leaves and lack physical protection. These high levels are also found in the soil around coffee seedlings, which prevents nearby seedlings from growing. This gives seedlings with the most caffeine fewer competitors for resources like water and nutrients. In tea leaves, caffeine is stored in two areas. First, in cell vacuoles, where it combines with polyphenols. This caffeine may be released into the mouthparts of insects to discourage them from eating the plant. Second, around the vascular bundles, where it likely stops harmful fungi from entering and growing in these areas. Caffeine in nectar may help plants reproduce by improving the memory of pollinators, such as honey bees, about the reward of visiting the plant.

The different effects of drinking caffeine-containing beverages from various plants may be due to the presence of other compounds, such as methylxanthine alkaloids like theophylline and theobromine, and polyphenols that can form insoluble complexes with caffeine.

Products

Products that contain caffeine include coffee, tea, soft drinks ("colas"), energy drinks, other beverages, chocolate, caffeine tablets, other oral products, and inhalation products. A 2020 study in the United States found that coffee is the main source of caffeine for middle-aged adults, while soft drinks and tea are the main sources for adolescents. Energy drinks are more commonly consumed by adolescents than by adults as a source of caffeine.

The world’s main source of caffeine is the coffee "bean," which is the seed of the coffee plant. Coffee is made by brewing these beans. The amount of caffeine in coffee depends on the type of bean and how it is prepared. Even beans from the same plant can have different caffeine levels. In general, one serving of coffee contains between 80 and 100 milligrams of caffeine. For example, a single shot (30 milliliters) of arabica-variety espresso has about 80 to 100 milligrams, while a cup (120 milliliters) of drip coffee has about 100 to 125 milligrams. Arabica coffee usually has about half the caffeine of robusta coffee. Dark-roast coffee has slightly less caffeine than lighter roasts because the roasting process reduces caffeine levels slightly.

Tea has more caffeine than coffee by dry weight. However, a typical serving of tea contains less caffeine than an equivalent serving of coffee because less tea is used. Other factors, such as growing conditions and processing methods, also affect caffeine levels. Therefore, teas can have varying amounts of caffeine.

Tea also contains small amounts of theobromine and slightly more theophylline than coffee. How tea is prepared and other factors greatly affect its caffeine content. The color of tea is not a good indicator of caffeine levels. For example, pale Japanese green tea like gyokuro contains more caffeine than darker teas like lapsang souchong, which has very little caffeine.

Soft drinks, such as cola, originally got their caffeine from kola nuts. These drinks usually contain 0 to 55 milligrams of caffeine per 12-ounce (350 mL) serving. Energy drinks, like Red Bull, can have as much as 80 milligrams of caffeine per serving. The caffeine in these drinks comes from ingredients, from the decaffeination process, or from chemical synthesis. Guarana, a key ingredient in energy drinks, has large amounts of caffeine and small amounts of theobromine and theophylline in a naturally occurring slow-release substance.

• Maté is a popular drink in parts of South America. It is made by filling a gourd with yerba mate leaves, pouring hot (but not boiling) water over the leaves, and drinking through a straw called a bombilla, which filters out the leaves.
• Guaraná is a soft drink from Brazil made from the seeds of the guaraná fruit.
• Ilex guayusa leaves from the Ecuadorian holly tree are boiled to make guayusa tea.
• Ilex vomitoria leaves from the yaupon holly tree are boiled to make yaupon tea.
• In Australia, commercially prepared coffee-flavored milk beverages, such as Oak's Ice Coffee and Farmers Union Iced Coffee, are popular. These drinks can have widely varying amounts of caffeine, which may differ from the amounts listed by manufacturers.

Cocoa solids, which come from cocoa beans, contain 230 milligrams of caffeine per 100 grams. The caffeine content varies between cocoa bean types. Caffeine content in milligrams per gram (sorted by lowest caffeine content):

• Forastero (defatted): 1.3 mg/g
• Nacional (defatted): 2.4 mg/g
• Trinitario (defatted): 6.3 mg/g
• Criollo (defatted): 11.3 mg/g

• Dark chocolate (70–85% cacao solids): 80 mg
• Dark chocolate (60–69% cacao solids): 86 mg
• Dark chocolate (45–59% cacao solids): 43 mg
• Milk chocolate: 20 mg

The stimulating effect of chocolate may come from a combination of theobromine, theophylline, and caffeine.

Caffeine tablets offer advantages over coffee, tea, and other caffeinated drinks, such as convenience, known amounts of caffeine, and avoiding sugar or acids. These tablets are often used by students studying for exams and by people who work or drive for long periods.

One US company sells dissolvable caffeine strips. Another way to take caffeine is through SpazzStick, a caffeinated lip balm. Alert Energy Caffeine Gum was introduced in the United States in 2013 but was later removed after the FDA investigated the health effects of added caffeine in foods.

There is weak evidence that using caffeine mouthwashes might improve cognitive performance.

Like an e-cigarette, a caffeine inhaler may deliver caffeine or a stimulant like guarana through vaping. In 2012, the FDA warned a company marketing an inhaler because there was not enough safety information about inhaled caffeine.

• Some drinks mix alcohol with caffeine to create caffeinated alcoholic beverages. The stimulant effects of caffeine may hide the depressant effects of alcohol, making people less aware of how drunk they are. These drinks have been banned in some places due to safety concerns. The FDA has classified caffeine added to malt liquor as an "unsafe food additive."
• Ya ba is a drug that combines methamphetamine and caffeine.
• Painkillers like propyphenazone/paracetamol/caffeine mix caffeine with an analgesic.

History

In Chinese legend, the emperor Shennong, who is believed to have ruled around 3000 BCE, accidentally discovered tea when he noticed that certain leaves, when dropped into boiling water, created a fragrant and restorative drink. Shennong is also mentioned in Lu Yu's Cha Jing, an early famous book about tea.

The earliest proof that people knew about coffee or drank it comes from around the middle of the 1500s, in Sufi monasteries in Yemen, southern Arabia. From Mokha, coffee spread to Egypt and North Africa, and by the 1600s, it reached the rest of the Middle East, Persia, and Turkey. Coffee drinking then spread to Italy, then to the rest of Europe, and coffee plants were brought to the East Indies and the Americas by the Dutch.

Kola nuts have been used for a long time. In many West African cultures, people chew kola nuts in both private and social settings to gain energy and reduce hunger.

The earliest proof of cocoa bean use comes from residue found in an ancient Mayan pot dating to 600 BCE. Chocolate was also consumed as a bitter and spicy drink called xocolatl, often mixed with vanilla, chile pepper, and achiote. Xocolatl was believed to help reduce tiredness, likely because of the theobromine and caffeine it contains. Chocolate was a valuable item in pre-Columbian Mesoamerica, and cocoa beans were sometimes used as money.

Xocolatl was brought to Europe by the Spaniards and became a popular drink by 1700. The Spaniards also introduced the cacao tree to the West Indies and the Philippines.

Native Americans used the leaves and stems of the yaupon holly (Ilex vomitoria) to brew a tea called asi or the "black drink." Evidence of this practice has been found from very ancient times, possibly dating back to the Late Archaic period.

In 1819, a German chemist named Friedlieb Ferdinand Runge first isolated pure caffeine and called it "Kaffebase" (a base found in coffee). In 1821, caffeine was also isolated by the French chemist Pierre Jean Robiquet and by another pair of French chemists, Pierre-Joseph Pelletier and Joseph Bienaimé Caventou, as reported by the Swedish chemist Jöns Jacob Berzelius in his yearly journal. Berzelius noted that the French chemists worked independently of each other and of Runge. However, Berzelius later acknowledged that Runge had discovered caffeine first.

Pelletier's article was the first to use the term "caféine" (from the French word for coffee: café) in print. It supports Berzelius's account: Robiquet was among the first to isolate and describe pure caffeine, while Pelletier was the first to analyze its chemical elements.

In 1827, M. Oudry isolated "théine" from tea, but in 1838, it was proven by Mulder and Carl Jobst that théine was the same as caffeine.

In 1895, the German chemist Hermann Emil Fischer first created caffeine from its chemical parts (a "total synthesis") and later determined its structure. This work contributed to Fischer receiving the Nobel Prize in 1902.

Because people knew coffee had a stimulant, coffee and caffeine have sometimes been regulated. For example, in the 1600s, Islamists in Mecca and the Ottoman Empire banned coffee for certain groups. In 1676, Charles II of England tried to ban coffee, and in 1777, Frederick II of Prussia banned it. Coffee was also banned in Sweden at various times between 1756 and 1823.

In 1911, caffeine became the focus of one of the earliest health scares when the U.S. government seized 40 barrels and 20 kegs of Coca-Cola syrup in Chattanooga, Tennessee, claiming the caffeine in the drink was "injurious to health." Although the Supreme Court later ruled in favor of Coca-Cola in United States v. Forty Barrels and Twenty Kegs of Coca-Cola, two bills were introduced in 1912 to the U.S. House of Representatives to add caffeine to the list of "habit-forming" and "deleterious" substances in the Pure Food and Drug Act, requiring it to be listed on product labels.

Society and culture

The U.S. Food and Drug Administration (FDA) says drinks with less than 0.02% caffeine are safe. However, caffeine powder sold as a dietary supplement is not controlled by the FDA. Most prepackaged foods must list their ingredients in order of how much is used, including additives like caffeine. But there is no rule that requires labels to show the exact amount of caffeine (such as milligrams per serving). Some natural ingredients, like coffee or chocolate, contain caffeine and must be listed on food labels. However, there is no rule that requires labels to state how much caffeine is in foods made from these natural sources. While coffee and chocolate are well-known for containing caffeine, other ingredients like guarana and yerba maté may be less familiar as caffeine sources. For these natural sources, there is no rule that requires labels to mention caffeine or its amount. The FDA updated its guidance on this topic in 2018.

Worldwide, about 120,000 metric tons of caffeine are consumed each year, making it the most popular psychoactive substance globally. Caffeine use has stayed about the same from 1997 to 2015. Coffee, tea, and soft drinks are the main sources of caffeine, while energy drinks contribute little to overall caffeine intake across all age groups.

The Seventh-day Adventist Church once asked members to avoid caffeinated drinks, but this rule was removed from baptismal vows, though the church still encourages avoiding caffeine as a policy. Some members believe caffeine is not meant to be used unless for medical reasons or because it is addictive. The Church of Jesus Christ of Latter-day Saints does not mention caffeine in its health guidelines, which prohibit alcohol, tobacco, and "hot drinks" (interpreted as tea and coffee).

Gaudiya Vaishnavas generally avoid caffeine because they believe it affects the mind and overstimulates the senses. To be accepted as a student under a spiritual teacher, individuals must avoid caffeine, alcohol, nicotine, and other drugs for at least one year.

Many Muslims consume caffeinated beverages. In the 16th century, some Muslim leaders tried to ban them as "intoxicating" under Islamic dietary laws, but these efforts were not successful.

Other organisms

The bacteria Pseudomonas putida CBB5 can survive by using pure caffeine as a food source. It can break down caffeine into carbon dioxide and ammonia.

Caffeine is harmful to birds, dogs, and cats. It also has a strong negative effect on mollusks, various insects, and spiders. This is partly because these animals have difficulty breaking down caffeine, which leads to higher amounts of caffeine in their bodies per unit of body weight. Caffeine has also been found to enhance the reward memory of honey bees.