Caffeine

Caffeine is a naturally occurring methylxanthine alkaloid found in the seeds, leaves, and fruits of more than 60 plant species, including coffee beans (Coffea spp.), tea leaves (Camellia sinensis), cocoa beans, kola nuts, guarana berries, and yerba maté. It is the most widely consumed psychoactive substance in the world, and it acts principally as a central nervous system stimulant through competitive antagonism of adenosine receptors, an action that reduces the perception of fatigue and increases alertness and arousal.¹² Beyond dietary sources, caffeine is sold in anhydrous form in dietary supplements, energy drinks, pre-workout products, and over-the-counter alertness aids, and it is used in pharmaceutical preparations for selected clinical indications such as apnea of prematurity in neonates and as an adjunct in some analgesic formulations.

Caffeine is absorbed rapidly and almost completely from the gastrointestinal tract, with peak plasma concentrations typically reached within 30 to 120 minutes of ingestion. The average elimination half-life in healthy adults is approximately 5 hours but varies substantially between individuals — from roughly 2 to 10 hours — based on genetic variation in the CYP1A2 enzyme that metabolizes caffeine, pregnancy status, smoking, hormonal contraceptive use, liver function, and concurrent medications.²

Caffeine is used for two broad categories of purposes: enhancement of cognitive performance (alertness, vigilance, sustained attention, particularly in conditions of sleep deprivation), and enhancement of exercise and athletic performance.

The 2021 International Society of Sports Nutrition (ISSN) position stand on caffeine and exercise performance, which evaluated the available randomized controlled trial literature, concluded that caffeine supplementation acutely enhances multiple aspects of exercise performance in many but not all studies, with small-to-moderate benefits documented across muscular endurance, movement velocity, muscular strength, sprinting, jumping, throwing, and a broad range of aerobic and anaerobic sport-specific actions. Aerobic endurance was identified as the form of exercise with the most consistent moderate-to-large effects.² The NIH Office of Dietary Supplements (ODS) fact sheet on exercise and athletic performance similarly summarizes caffeine as one of the better-supported ergogenic ingredients, while noting that effect magnitude varies between individuals and across study designs.³

For cognitive performance, caffeine has been studied extensively for effects on alertness, reaction time, vigilance, and mood, particularly under conditions of partial sleep deprivation. The ISSN position stand notes that caffeine can enhance vigilance during bouts of extended exhaustive exercise and during periods of sustained sleep deprivation.²

The U.S. Food and Drug Administration has cited 400 mg per day as an amount of caffeine not generally associated with negative effects in most healthy adults, approximating two to three 12-fluid-ounce cups of brewed coffee. The agency notes that individual sensitivity varies substantially based on body weight, medications, pregnancy status, and certain medical conditions, and that no specific level has been established for children.¹

For exercise performance specifically, the ISSN reports that the most commonly studied effective range is 3 to 6 mg per kg body mass, typically consumed approximately 30 to 60 minutes before exercise, with no further enhancement observed at doses of 9 mg/kg or higher.² Alternative delivery formats including caffeinated chewing gum, mouth rinses, energy gels, and chews have also been studied and may produce performance effects, particularly for aerobic exercise.²

Approximate caffeine content of common products varies widely. The FDA notes that a typical 8-ounce cup of brewed coffee contains roughly 80 to 100 mg, a 12-ounce can of caffeinated soft drink roughly 30 to 40 mg, an 8-ounce cup of green or black tea roughly 30 to 50 mg, and an 8-ounce energy drink roughly 40 to 250 mg. Pure caffeine tablets and pre-workout supplements vary widely; product labels are the most reliable source of per-serving content.¹

The American College of Obstetricians and Gynecologists has stated that moderate caffeine consumption (less than 200 mg per day) does not appear to be a major contributing factor in miscarriage or preterm birth; pregnant individuals should consult their healthcare provider regarding individual circumstances. The American Academy of Pediatrics recommends against energy drinks for children and adolescents because of the levels of sugar and caffeine.¹

Oral, in beverages, tablets, capsules, chewing gum, mouth rinses, gels, and dissolvable strips. In selected clinical settings, caffeine can also be administered intravenously.

Standard storage in a cool, dry place out of direct sunlight. Pure powdered or highly concentrated liquid caffeine carries a particular safety concern: a single teaspoon of pure powdered caffeine is roughly equivalent to the caffeine content of about 28 cups of regular coffee, and accidental over-ingestion has been associated with deaths. The FDA has issued guidance and taken enforcement action restricting the bulk sale of pure and highly concentrated caffeine directly to consumers.¹

Common adverse effects of caffeine include insomnia, anxiety, restlessness, gastrointestinal upset, tachycardia, tremor, and elevated blood pressure. These effects are dose-related and vary substantially by individual sensitivity.¹²

Several practical considerations apply:¹²

• Sleep: Caffeine consumed even 6 hours before bedtime can measurably disrupt sleep in some individuals, given its long elimination half-life.
• Medication interactions: Caffeine interacts with a variety of medications including some antibiotics (notably ciprofloxacin), some antidepressants (notably fluvoxamine), and theophylline. Individuals taking multiple medications may benefit from consultation with a pharmacist.
• Pregnancy and breastfeeding: Caffeine crosses the placenta and enters breast milk; consensus guidance is to limit intake during pregnancy and lactation.
• Cardiovascular and arrhythmia history: Individuals with cardiovascular disease, hypertension, or arrhythmia history should discuss caffeine intake with their clinician.
• Anxiety disorders: Caffeine can exacerbate anxiety symptoms and may worsen panic-prone presentations.
• Withdrawal: Regular caffeine consumers may experience withdrawal symptoms — headache, fatigue, irritability, low mood — when intake is abruptly reduced; gradual tapering avoids most of these symptoms.

Caffeine tolerance develops with regular use, and habitual consumers may experience smaller acute performance effects than caffeine-naive individuals, although the ISSN notes that ergogenic benefits do not appear to be entirely abolished by habituation in most studies.²