Caffeine

Caffeine is a bitter white powder. It is a methylxanthine, the family of plant alkaloids that also holds theobromine and theophylline. Chemically it is close kin to adenine and guanine, two of the bases in DNA.

Plants make it in the seeds, fruits and leaves of coffee, tea, cacao and guarana. It is the most widely used psychoactive (mind-altering) drug in the world, and more than 80% of US adults consume it.

Plants did not make it for us. Caffeine occurs naturally in the nectar of coffee and citrus flowers. Honeybees that were rewarded with it were three times as likely to remember a flower’s scent. It acts on the bees’ memory cells much as it acts on ours, by blocking adenosine receptors, so the plant buys itself a loyal pollinator.

Most readers do not know how much of the morning lift is the end of a mild withdrawal. In two trials from one research group, regular users who went without overnight felt less alert. A dose of caffeine only brought them back to baseline.

People who had been off caffeine for three weeks gained no clearer thinking from it. The amount in a cup also swings. The same drink from the same coffee shop held from 259 to 564 mg over six days in a row.

Health benefits of caffeine

• ★★★Better exercise performanceAcross 21 meta-analyses, caffeine improved aerobic endurance, strength, power and speed, most for aerobic exercise. In 46 studies of endurance time trials at 3 to 6 mg/kg, finishing times were 2.22% faster.
• ★★★Better attention and reactions when short of sleepIn people who were sleep-deprived or sleep-restricted (45 publications), caffeine improved accuracy on attention tests, reaction time and vehicle control on driving tests. Memory, physical and job-related performance also usually improved.
• ★★★Shorter and lighter sleepAcross 24 studies, caffeine cut total sleep time by 45 minutes and deep sleep by 11.4 minutes. In another trial, 400 mg disturbed sleep even when taken 6 hours before bedtime.
• ★★★Higher blood pressure with regular caffeineRegular caffeine (median 410 mg a day) raised the top blood pressure number by about 4 points (4.16 mm Hg) across 16 trials. Coffee raised it by about 1 point (1.22 mm Hg), a small effect.
• ★★★Slightly better pain relief when added to painkillersAdding 100 to 130 mg of caffeine to paracetamol or ibuprofen gave 5% to 10% more people good relief (20 studies, 7,238 people). About 14 people must take the caffeine version for one extra person to benefit.

Foods high in caffeine

Coffee is the main source of caffeine for adults in most European surveys (40% to 94% of intake). Tea leads in Ireland and the United Kingdom, and chocolate leads for children aged 3 to 10 in most countries.

EFSA, the EU’s food safety agency, gives rough servings. An espresso (60 mL) has 80 mg, and a cup of filter coffee (200 mL) 90 mg. A cup of black tea (220 mL) has 50 mg, and a can of cola (355 mL) 40 mg.

A can of energy drink (250 mL) has 80 mg. A 50 g bar of plain chocolate has 25 mg, or 10 mg for milk chocolate. Health Canada lists 25 to 58 mg in an ounce (28 g) of unsweetened baking chocolate. Caffeine’s relatives theobromine and theophylline have their own pages.

Real cups vary a lot. In a lab test of specialty coffees from coffee shops, a serving held from 58 to 259 mg. The average was 188 mg in a 16 ounce cup. The same drink from the same shop varied from 259 to 564 mg over six days.

Brewed white, green and black teas held 14 to 61 mg per 6 or 8 ounce serving, and steeping time changed the amount. Decaffeinated coffee is not caffeine-free: the FDA (the US Food and Drug Administration) says typically 2 to 15 mg in an 8 ounce cup. The ranking below uses USDA FoodData Central values.

How caffeine works

Most of what we know about how caffeine works comes from animals, cells and small drug studies in people. Four ideas explain most of what you feel. It blocks adenosine receptors, your liver sets how long it lasts, and the wake-up effect also costs sleep. Plants, for their part, use it as a drug for insects.

It blocks adenosine receptors

Caffeine’s main action in the brain is to block adenosine receptors. A major review of its effects on performance says this is how it exerts its effects. Alertness, vigilance, attention and reaction time improve after low to moderate doses (about 40 to 300 mg).

In mice, the wakefulness caused by caffeine needed the A2A type of receptor and did not need the A1 type. A review lists two further actions: freeing calcium inside cells and blocking enzymes called phosphodiesterases.

People differ at this receptor. Carriers of one ADORA2A variant (rs5751876 TT) felt more anxiety after caffeine. Regular users built up tolerance to that anxiety, whatever their genes.

Your liver sets how long it lasts

An enzyme called CYP1A2 clears about 95% of the caffeine you take in, and people differ in how fast it works. In one small study, smokers cleared caffeine faster. Their half-life, the time to clear half of it, was 3.5 hours against 6.0 in nonsmokers, probably because smoking switches on liver enzymes.

Quitting smoking slowed clearance in another study. Drugs that block the enzyme do the opposite. With fluvoxamine, caffeine’s half-life rose from 4.9 to 56 hours.

The wake-up effect also costs sleep

The same action that keeps you awake works at night too. A meta-analysis of 24 studies found caffeine cut total sleep time by 45 minutes and sleep efficiency by 7%. The authors advise stopping coffee at least 8.8 hours before bed.

Plants use it as a drug for insects

Caffeine is a defense chemical in many plants. Honeybees rewarded with it were three times as likely to remember a flower scent, which suggests the plant uses it to bring pollinators back. The bees’ memory cells responded because caffeine blocked adenosine receptors, the same target it has in people.

What has been seen in people

The mechanisms fit the human trials above. In people who had lost sleep, a meta-analysis found better attention, reaction time and driving-test control.

In regular users the picture is less simple. After three weeks off caffeine, a dose brought no better thinking. In a larger trial, the alertness caffeine added mostly undid the lag of going without.

At night the cost is real: 45 minutes of sleep in 24 studies. Over days, trials of caffeine raised systolic blood pressure by about 4 mmHg, less when it came as coffee. Genes matter for some results: in 101 athletes, only those with the AA form of CYP1A2 gained speed.

An umbrella review of caffeine meta-analyses also linked it to lower Parkinson’s and type 2 diabetes risk and higher pregnancy loss risk. Those are observational findings.

Is caffeine safe? Supplements, dose and interactions

For most healthy adults, coffee and tea are not a safety concern at ordinary amounts. EFSA found that up to 400 mg a day raises no safety concern for healthy adults except pregnant women. That is about 5.7 mg per kg of body weight. Single doses up to 200 mg (about 3 mg/kg) do not either.

The FDA cites 400 mg a day, about two to three 12 ounce cups of coffee, as an amount not generally linked to negative effects. Health Canada also gives 400 mg for adults. EFSA adds that a single 100 mg dose may affect sleep in some adults when taken near bedtime.

A 2017 systematic review of 381 studies reached the same 400 mg conclusion for healthy adults, and 300 mg for healthy pregnant women. It was funded by industry groups including the National Coffee Association and the American Beverage Association.

• Doses and side effects. The ISSN says 3 to 6 mg/kg works for exercise and that 9 mg/kg brings many side effects. The FDA lists signs of too much: a fast heartbeat, palpitations, high blood pressure, insomnia, anxiety, jitters, upset stomach, nausea and headache. It says toxic effects such as seizures can occur with about 1,200 mg taken quickly, which is less than half a teaspoon of pure caffeine. Pure and concentrated caffeine products can be fatal, it adds. People with panic disorder had attacks after 400 to 750 mg.
• Pregnancy, breastfeeding and children. ACOG, the US obstetricians’ college, says less than 200 mg a day does not appear to be a major factor in miscarriage or preterm birth. In Committee Opinion 462, reaffirmed in 2026, it calls the link to growth restriction undetermined. EFSA also gives 200 mg a day for pregnant and breastfeeding women, and Health Canada gives 300 mg. For children and teens, EFSA gives 3 mg/kg a day and Health Canada 2.5 mg/kg. The FDA says caffeinated drinks should be avoided under age 2.
• Heart and blood pressure. In a 2026 American Heart Association statement, trials in regular coffee drinkers showed a lower risk of atrial fibrillation but more premature ventricular beats. Data on high doses, such as energy drinks, are limited and generally suggest harm. If you have a heart rhythm condition or high blood pressure, ask your clinician.
• Drugs, smoking and genes. Fluvoxamine raised caffeine’s half-life from 4.9 to 56 hours, and ciprofloxacin from 5.2 to 8.2 hours. A review also lists theophylline and other CYP1A2 blockers. Smokers clear caffeine faster, and clearance slowed when smokers quit. Variants in the CYP1A2 and ADORA2A genes changed athletic and anxiety responses in the trials above. But we found no guideline that uses genetic tests.

How to get more caffeine

• Pick the source for the dose you want. EFSA puts filter coffee at about 90 mg and black tea at 50 mg a cup. But one coffee shop’s drink ranged from 259 to 564 mg on different days, so check labels and count every source.
• Time it. For exercise the ISSN uses 60 minutes before. For sleep, a meta-analysis advises a coffee at least 8.8 hours before bed, and a trial found 400 mg disturbed sleep even 6 hours before.
• Cut back slowly. The FDA says gradual reduction is best. Withdrawal headaches start 12 to 24 hours after the last dose and symptoms last 2 to 9 days, so a gradual taper is easier.

History of caffeine

The first uses of coffee and tea are dressed in myth. A 2011 history says coffee became widespread in the fifteenth and sixteenth centuries and reached Europe in the eighteenth and nineteenth. Tea became common during the Ming dynasty and in eighteenth-century Britain. Both were first used as medicines before they became daily drinks.

Caffeine itself was found in 1819. The German chemist Friedlieb Ferdinand Runge had impressed the poet Goethe with a demonstration of belladonna. Goethe handed him a box of coffee beans to study, and soon after, Runge found caffeine in them. He called it Kaffebase.

In 1821 the French chemists Robiquet, and Pelletier with Caventou, isolated it independently. Pelletier’s article was the first to print the French word caféine. The Swedish chemist Berzelius later acknowledged that Runge had been first.

In 1895 Hermann Emil Fischer made caffeine from simple chemicals for the first time. He worked out its structure two years later. He won the 1902 Nobel Prize in Chemistry for his work on sugar and purine syntheses.

In 2015 EFSA published the first assessment of caffeine from all dietary sources at EU level. In the mid-2010s the FDA took action against pure and highly concentrated caffeine products. In 2026 the American Heart Association issued a scientific statement on caffeine and cardiovascular disease.

Frequently asked questions about caffeine

What does caffeine do for the body?

It makes you more alert by blocking adenosine receptors in the brain. A meta-analysis of 45 publications found better attention, reaction time and driving-test control in people who had lost sleep. In 21 meta-analyses, exercise performance was better. It also raises blood pressure a little (4.16 mmHg systolic in trials of caffeine pills) and costs sleep. In regular users, part of the lift is the end of withdrawal.

How much caffeine is too much in a day?

For most healthy adults, 400 mg a day is the figure EFSA, the FDA and Health Canada use. That is about two to three 12 ounce cups of coffee by the FDA’s count. Single doses above 200 mg are not covered by EFSA’s no-concern level. The FDA says about 1,200 mg taken quickly, less than half a teaspoon of pure caffeine, can cause seizures. Sensitivity varies a lot with body weight, drugs and genes.

Which has more caffeine, coffee or tea?

Coffee usually does. EFSA lists about 90 mg for a cup of filter coffee and 50 mg for a cup of black tea. One study found brewed teas held 14 to 61 mg per serving. Cups vary: one coffee shop’s drink ranged from 259 to 564 mg over six days. Plain chocolate has about 25 mg in a 50 g bar.

Is caffeine safe during pregnancy?

In small amounts, it is generally accepted. ACOG says less than 200 mg a day does not appear to be a major factor in miscarriage or preterm birth. EFSA gives 200 mg too, while Health Canada gives 300 mg. Meta-analyses of observational studies link higher intakes to low birth weight and pregnancy loss. But a trial in 1207 women found that cutting caffeine did not change birth weight. Talk to your doctor.

How long before bed should you stop caffeine?

A meta-analysis of 24 studies advises a coffee at least 8.8 hours before bed, and 13.2 hours for a pre-workout supplement. In a trial, 400 mg of caffeine disturbed sleep even 6 hours before bedtime. Caffeine cut total sleep time by 45 minutes on average. Smokers clear caffeine faster, and some drugs slow it a lot.

Does caffeine improve workouts?

Yes, a little. The ISSN says 3 to 6 mg/kg taken about 60 minutes before exercise consistently helps, with the most consistent gains in endurance. A meta-analysis of 46 studies found time trials 2.22% faster. Not everyone responds: in 101 athletes, only those with the CYP1A2 AA genotype got faster. Very high doses of 9 mg/kg bring side effects.

Can caffeine cause heart problems?

In the studies we found, ordinary amounts did not cause atrial fibrillation. A meta-analysis of 7 studies found no higher risk. In a 2026 trial of coffee drinkers having a cardioversion, those who kept drinking coffee had fewer relapses. Caffeinated coffee did raise premature ventricular beats from 102 to 154 a day. Energy drink doses are less studied, so ask your clinician if you have a heart condition.

How long does caffeine withdrawal last?

Usually 2 to 9 days. A review of 57 experiments found symptoms start 12 to 24 hours after the last dose and peak at 20 to 51 hours. They include headache in about half of people, plus fatigue, low mood and trouble concentrating. Going without as little as 100 mg a day caused symptoms. Cutting back gradually, as the FDA advises, is gentler.