Caffeine is a psychoactive drug that exerts a powerful stimulant effect on the central nervous system. It is naturally found in coffee, tea, and chocolate but is also added to some soft drinks (e.g. colas), energy drinks and energy shots.^[1] The National Health and Nutrition Examination Survey (NHANES) found that roughly 89% of adults in the US consume caffeine daily.^[2] People often use caffeine-containing beverages to help restore alertness and wakefulness or to reduce drowsiness when feeling mentally fatigued.^[3][4] Caffeine is also available as a purified powder, and it is found in some dietary supplements claiming to cause weight loss or increase energy (e.g. pre-workouts).^[1][5][6] Additionally, caffeine is found in some over-the-counter drugs used to relieve pain^[7][8] and in prescription drugs used to treat breathing problems in premature babies.^[9]

After ingestion, caffeine acts rapidly on the central nervous system, triggering several short-lived effects including an increase in heart rate, blood pressure, and urine output.^{[10][11][12][13]} However, with regular daily use, these symptoms of caffeine typically become less pronounced.^{[10][12][1][3][4]}

Caffeine restores alertness and wakefulness, and reduces drowsiness during mental fatigue.^[14][15][16] It also improves several aspects of cognitive function — accuracy, reasoning, memory, reaction time, attention, etc. — during cognitively and/or physically demanding tasks.^{[17][14][15][16]} Furthermore, caffeine can improve some aspects of cognitive function that have been impaired by acute sleep deprivation.^[15][16]

Caffeine can also help with pain relief. For example, when caffeine is added to analgesic (pain relieving) drugs like acetaminophen (also known as paracetamol) or ibuprofen, there is a small but clinically meaningful improvement in pain.^[7][8]

In hospitals, caffeine (typically caffeine citrate) is used to treat apnea (temporary cessation of breathing) in premature babies but the optimal dosing strategy and it’s effect on survival and subsequent neurocognitive development are unclear.^[9] Furthermore, in people with asthma, caffeine may have a small beneficial effect on some aspects of respiratory function (forced expiratory volume in one minute, FEV1, and mid-expiratory flow rate) for up to four hours.^[18] However, this effect also has implications for asthma diagnostics because prior caffeine ingestion may reduce the accuracy of lung function tests.^[18]

Caffeine can raise whole-body fat oxidation rates (at rest and during exercise)^[19][20] and increase daily energy expenditure^[21][22] but the effects are negligible — approximately 400 kilojoules or 100 kilocalories per day. The effect of caffeine on appetite regulation and energy intake is less clear and varies depending on timing and dose, etc.^[22][23] Consequently, the role of caffeine in weight loss and/or weight management is unclear. Despite one meta-analysis concluding that caffeine can promote reductions in weight, BMI, and body fat,^[24] its utility is limited because the included trials used energy-restriction diets and/or ephedrine, a powerful stimulant that causes weight loss.

Caffeine is widely used in sports because it can improve both aerobic and anaerobic performance in trained and untrained individuals.^[25] For example, when taken before or during exercise, caffeine has small to moderate-sized effects to lower the rating of perceived exertion (RPE)^[26][27] and improve aerobic endurance, anaerobic power, sprint speed, muscle endurance, muscle strength, muscle power (jump height), and agility.^[25][28][29] These effects are found across a range of sports (running, cycling, and swimming, etc.) including skill-based team sports (e.g. basketball, soccer, etc.).^[25][30][31] Furthermore, when taken before and/or during exercise, caffeine may also improve cognitive functions, such as attention, reaction time, memory, and feelings of fatigue.^[14] One drawback is the predominance of research studies only including young male participants.^[28][25] Some meta-analyses find similar performance benefits in females as those found in males^[32][33][34] but more randomized controlled trials in females are needed to improve the generalization of sports nutrition recommendations for caffeine.^[25][35]

Caffeine is generally safe for most people if used within the recommended amounts (up to 400 mg per day in a healthy adult or up to 200 mg in a single dose — see the dosage info section.^[36] Furthermore, caffeine-containing beverages like tea and coffee are socially acceptable and widely used.^[37][2] However, caffeine has several side effects and drawbacks.

The side effects of caffeine include a short-lived rise in heart rate and blood pressure, heart palpitations, headache, increased urine output, nervousness, gastrointestinal problems, etc.^{[10][11][12][13]} Caffeine can also raise heart rate during and following exercise.^[38][39] Some of these side effects, particularly those related to heart rate, blood pressure, and urine output, may subside with regular use due to increased tolerance.^{[10][12][1][3][4]} Because caffeine can cause mild drug dependence, some people also experience withdrawal symptoms — headache, drowsiness, and irritability — when they stop using caffeine after regular daily intake.^[40]

One of the major drawbacks is that caffeine impairs sleep quality and can reduce sleep duration.^[41][42] A recent meta-analysis found that to prevent deleterious effects on subsequent sleep, coffee should be consumed at least 9 hours before bedtime and a caffeine-containing pre-workout supplement should be consumed at least 13 hours before bedtime.^[41]

Sadly, there have been several case reports of caffeine toxicity^{[43][44][45][46]} and caffeine-related death.^{[47][48][49][50]} However, given the high global prevalence of caffeine intake, these cases are rare.^[37][2][51] Such cases also typically involve exceptionally high caffeine intakes (usually unintentional but sometimes purposeful) or caffeine intake combined with other drugs and alcohol.^[47] Some instances of toxicity and death involve energy drinks — beverages that contain as much as 300 mg of caffeine plus other stimulants like taurine, plus B vitamins, and sugar.^[52] At the population level, energy drinks are unlikely to cause harm because there is a low prevalence of consumption.^[53][54] However, in certain groups (children, teenagers, people with underlying heart conditions), health risks can emerge when people far exceed recommended daily caffeine intake limits by consuming energy drinks with other caffeine-containing foods and beverages.^[53][54]

Some epidemiological studies show a relationship between high daily caffeine intake (e.g. more than 4 to 6 cups of coffee per day) and increased cardiovascular disease risk.^{[55][56][57][58]} However, the evidence from mendelian randomization studies and meta-analyses of epidemiological studies shows that caffeine (or coffee) intake does not increase the risk of cardiovascular disease, including coronary artery disease, stroke, heart failure, and cardiovascular disease mortality^{[54][58][59][60]} and that moderate intake might even be protective.^{[61][62][63][64]}

Some studies also find a relationship between caffeine intake during pregnancy and poorer health outcomes for mother and baby following birth; however, the entirety of the evidence is equivocal.^[65][66] Furthermore, it is unclear whether reducing caffeine intake during pregnancy has any effect on pregnancy outcomes, including infant health and birth weight.^[65][66] However, some authorities recommend that pregnant women limit their caffeine intake to 200 milligrams per day (equivalent of 1–2 cups of coffee per day or less).^[67]

Caffeine’s mechanisms of action are not completely understood, but the proposed mechanisms likely act in unison.^[4][25][68]

Caffeine’s primary action is in the central nervous system, where it prevents adenosine binding to its receptor.^[69][4][68] This affects the secretion of several neurotransmitters — norepinephrine, dopamine, acetylcholine, serotonin, GABA, etc. — involved in alertness, mood, motivation, memory, and pain perception.^[4][70] For example, adenosine released during exercise decreases the secretion of dopamine, a neurotransmitter that increases mood and motivation.^[25][71] So, when taken before or during exercise, caffeine allows dopamine secretion to continue, potentially maintaining mood and motivation to work hard.^[71] This coupled with lower pain perception caused by the lack of adenosine signaling may partly explain caffeine’s sports performance-enhancing effects.

Caffeine also plays a role in cellular calcium transport, which regulates neurotransmitter secretion in neurons and contractile force in muscle cells.^[4][68] Caffeine can have a direct effect on calcium transport in muscle cells and may, therefore, have a direct effect on muscle contractile force,^{[25][72][73][74][75][76]} providing an additional explanation for its performance-enhancing effects.

Caffeine can also inhibit an enzyme called cyclic nucleotide phosphodiesterase, preventing the breakdown of cyclic adenosine monophosphate (cAMP).^[4][68] This is important because cAMP stimulates the release of neurotransmitters like dopamine, epinephrine, and norepinephrine, which regulate aspects of cognitive function, including alertness, mood, motivation, and memory.^[4][68] However, this mechanism requires more research and may only occur in the presence of high caffeine doses.