Caffeine

non-nutrient/non-botanical

What is it

Caffeine (1,3,7-trimethylxanthine) is the most widely consumed psychoactive substance in the world. It is a naturally occurring methylxanthine alkaloid found in coffee, tea, cocoa, yerba mate, guarana, and kola nuts, and is also added to many beverages, supplements, and medications.

How it works

Caffeine works mainly by blocking adenosine receptors in the brain. Adenosine accumulates with wakefulness and promotes sleepiness; when its receptors are blocked, the buildup signal is dampened, leading to greater alertness, faster reaction time, and reduced perceived fatigue. Caffeine also increases the release of dopamine and norepinephrine, contributing to mood elevation and arousal. In the body, caffeine raises basal metabolic rate by 3-11%, mobilizes fatty acids, and produces mild increases in heart rate and blood pressure. It is rapidly absorbed (peak blood levels 30-60 minutes after ingestion) and has a half-life that varies from 3 to 7 hours depending on genetics (CYP1A2 activity), smoking status, pregnancy, and certain medications.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Alertness and cognitive performance

Grade A

Strong evidence

Numerous randomized trials show caffeine improves alertness, reaction time, attention, vigilance, and reduces perceived fatigue. Effects are most pronounced in sleep-deprived states and at doses of 100-200 mg.

Athletic performance

Grade A

Strong evidence

Caffeine is one of the best-studied ergogenic aids. Meta-analyses consistently show small to moderate improvements in endurance, time-trial performance, strength, and power output at doses of 3-6 mg per kg.

Headache and migraine relief

Grade B

Good evidence

Caffeine enhances the analgesic effects of common pain relievers and is included in many headache medications. It can be useful for tension headaches and migraines, though overuse can cause rebound headaches.

Weight management

Grade C

Moderate evidence

Caffeine modestly increases metabolic rate and fat oxidation acutely, but tolerance develops quickly. Long-term effects on weight are small and often offset by adaptations.

Reduced risk of certain chronic diseases (coffee studies)

Grade C

Moderate evidence

Long-term observational research links regular coffee consumption to reduced risk of type 2 diabetes, Parkinson's disease, and certain liver conditions. It is unclear how much of this is attributable to caffeine versus other compounds in coffee.

4 commercial forms

Caffeine anhydrous

Rapidly absorbed; high bioavailability

Pure crystalline caffeine used in pills, capsules, and pre-workouts. Provides precise dosing.

Caffeine citrate

Similar absorption to anhydrous

A more water-soluble salt used in clinical settings and some supplements.

Natural caffeine (from coffee or tea extract)

Comparable to anhydrous when isolated

Sourced from coffee beans, tea, or guarana. May come with co-occurring compounds.

Caffeine in coffee, tea, or yerba mate

Absorption slightly slower than capsules

Beverage form. Effects come with additional polyphenols and other compounds.

Dosage

Most healthy adults can safely consume up to 400 mg of caffeine per day, equivalent to about 4 cups of brewed coffee. Single doses of 100-200 mg are typical for alertness; 3-6 mg per kg of body weight 30-60 minutes before exercise is the most studied performance dose. Doses above 500-600 mg per day are more likely to cause side effects.

When and how to take it

For alertness, take 30-60 minutes before needed effect. For exercise performance, consume 30-60 minutes before activity. Because of the long half-life (3-7 hours), avoid caffeine within 6-8 hours of bedtime to prevent sleep disruption. Pairing caffeine with a meal slightly slows absorption and may reduce stomach upset.

Food sources

Food	Amount	%DV
Brewed coffee (8 oz)	80-150 mg	—
Espresso (1 oz shot)	60-75 mg	—
Black tea (8 oz)	40-70 mg	—
Green tea (8 oz)	25-50 mg	—
Cola (12 oz)	30-45 mg	—
Energy drink (8 oz)	80-160 mg	—
Dark chocolate (1 oz)	10-25 mg	—

Safety

Common side effects include insomnia, jitteriness, anxiety, headache, increased heart rate, and gastrointestinal upset. Tolerance develops with regular use, and withdrawal symptoms (headache, fatigue, irritability) can occur after abrupt cessation. Acute overdose, more likely with concentrated supplements than beverages, can cause arrhythmias, seizures, and rarely death; the threshold is typically above 1.2 g.

Who should be cautious

People with arrhythmias, uncontrolled high blood pressure, anxiety disorders, panic disorder, gastric ulcers, glaucoma, or insomnia should limit caffeine. Pregnant women should keep intake under 200 mg per day. Children and adolescents should consume only minimal amounts. People with slow caffeine metabolism (variants of CYP1A2) may need lower doses.

Interactions

Caffeine is metabolized by CYP1A2 and can interact with many drugs. Ciprofloxacin, fluvoxamine, and oral contraceptives slow caffeine clearance. Caffeine reduces the effects of adenosine (used in cardiac testing) and benzodiazepines. It may interact with lithium, theophylline, MAO inhibitors, and stimulant medications. Combined use with ephedra-like compounds increases cardiovascular risk.

Frequently asked questions

How much caffeine is safe per day?⌄

Most healthy adults can safely consume up to 400 mg per day. Pregnant women should stay under 200 mg. People with heart conditions or anxiety may need less.

Why do I feel anxious from caffeine?⌄

Caffeine increases norepinephrine and can produce or worsen anxiety, especially at higher doses or in sensitive individuals. Genetics (CYP1A2, ADORA2A variants) influence sensitivity.

Does caffeine dehydrate me?⌄

At typical intakes, caffeine's mild diuretic effect does not lead to net dehydration. The water in coffee and tea contributes to daily fluid intake.

Why does caffeine work less over time?⌄

Tolerance develops as the brain compensates by producing more adenosine receptors. Taking caffeine breaks of 1-2 weeks can restore sensitivity.

How do I avoid sleep problems?⌄

Avoid caffeine within 6-8 hours of bedtime. Some people need an even longer cutoff. Effects on sleep can be subtle even when you fall asleep easily.

Is caffeine addictive?⌄

Caffeine causes physical dependence with regular use, leading to withdrawal symptoms on cessation. It is generally not considered addictive in the same way as drugs of abuse.

References

Caffeine (Wikidata) — Wikidata link
Caffeine (PubChem CID 2519) — PubChem link
Caffeine (ChEBI 27732) — ChEBI link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.